Objective lens system

ABSTRACT

A portable TV telephone includes an objective lens system. The objective lens system is configured to form an image of an object. An antenna is configured to transmit and receive at least one of said image of the object, a dial number and a voice carried by radio waves. A display is configured to display at least one of the image of the object and the dial number. The objective lens system includes a first negative lens unit and a second positive lens unit in order from an object side. An aperture stop is disposed between the first lens unit and said second lens unit. In one embodiment, the first lens and the second lens each have a lens barrel configured to position the respective lens units such that circumferences of the lens units contact interior walls of the portable TV telephone. In another embodiment, the first lens unit and the second lens unit are cemented at outer circumferential portions outside an effective diameter through which rays transmit.

This is a division of application Ser. No. 08/713,069, filed Sep. 12,1996 now U.S. Pat. No. 5,999,327.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to an objective lens system which uses aradial type gradient index lens element.

b) Description of the Prior Art

As conventional examples of lens systems having relatively wide fieldangles and high optical performance which are used, for example, asobjective lens systems for endoscopes, there are known many lens systemssuch as a lens system disclosed by Japanese Patent Kokoku PublicationNo. Sho 60-46410 shown in FIG. 1. A conventional example shown in FIG. 1is a lens system of the so-called retrofocus type which is composed, inorder from the object side, of a front lens unit having negativerefractive power, a stop and a rear lens unit having a positiverefractive power. In this lens system, the front lens unit has afunction to widen a field angle and another function to correctcurvature of field by reducing a Petzval's sum of the lens system as awhole, the rear lens unit serves for suppressing production of sphericalaberration and coma by distributing refractive powers among threepositive lens elements and a cemented lens component, and the cementedlens component corrects lateral chromatic aberration which poses aproblem, in particular, in a lens system which has a wide field angle.Though this conventional example favorably corrects aberrations, it iscomposed of lens elements in a number as large as six, thereby posing aproblem of low producibility or a high manufacturing cost. Accordingly,it is expected to develop a retrofocus type objective lens system forendoscopes which is composed of an extremely small number of lenselements and has a high producibility. However, a lens system whichfavorably corrects aberrations and has high optical performance canhardly be composed of two homogenous spherical lens elements.

Further, an endoscope is generally equipped with a system forilluminating a location to be observed since it is used frequently forobserving and photographing dark locations such as interiors of humanbodies, aircraft engines, pipings and so on. In addition, an objectivelens system for endoscopes has an NA which is not so large for obtaininga large depth of field. Accordingly, axial aberrations do not pose aserious problem, but a wide field angle and a negative-positiveasymmetrical composition as shown in FIG. 1 make it difficult to correctoffaxial aberrations. When an objective lens system is to be composed oftwo negative and positive lens elements, it is impossible to use acemented lens component as in the objective lens system disclosed byJapanese Patent Kokoku Publication No. Sho 60-46410 mentioned above asthe conventional example, whereby lateral chromatic aberration canhardly be corrected and offaxial imaging performance is remarkablydegraded. It is therefore difficult to favorably correct offaxialaberrations with two homogenous lens elements so as to obtain favorableoffaxial imaging performance.

It is therefore conceivable to use a radial type gradient index lenselement which is characterized in that it corrects chromatic aberrationin particular more favor- ably than a homogenous lens element. As aconventional example of an objective lens system for endoscopes whichuses a radial type gradient index lens element and is composed of twolens elements, there is known a lens system disclosed by Japanese PatentKokai Publication No. Sho 52-29238. However, this conventional examplehas a field angle as narrow as 72° which is insufficient for use as anobjective lens system for endoscopes.

Further, a lens system disclosed by Japanese Patent Kokai PublicationNo. Hei 5-107471,for example, is known as a conventional example of anobjective lens system having a wide field angle obtained with two lenselements. Though this example uses a radial type gradient index lenselement, it does not effectively make use of the chromatic aberrationcorrecting capability of the radial type gradient index lens element anddoes not sufficiently correct lateral chromatic aberration which poses aproblem in a lens system having a wide field angle in particular.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide an objectivelens system which is composed of lens elements in a number on the orderof 2, that favorably corrects aberrations and has a wide field angle.

The objective lens system according to the present invention ischaracterized in that it comprises, in order from the object side, afirst lens unit having a negative refractive power and a second lensunit having a positive refractive power; that at least the first lensunit comprises a radial gradient index lens element which has arefractive index distribution in a radial direction of the lens element;and that the first lens unit satisfies the following condition (1):

1/V ₁₀<1/V ₀₀  (1)

wherein the reference symbols V₀₀ and V₁₀ represent parametersexpressing a dispersing power of the radial type gradient index lenselement which are given by the following formulae (b) and (c)respectively:

V ₀₀=(N _(00d)−1)/(N _(00F) −N _(00C))  (b)

V ₁₀ =N _(10d)/(N _(10F) −N _(10C))

wherein the reference symbols N_(00d), N_(00F) and N_(00C) representrefractive indices of the radial type gradient index lens element forthe d-line, F-line and C-line respectively, and the reference symbolsN_(10d), N_(10F) and N_(10C) designate values of a coefficient of a termr² for the d-line, F-line and C-line respectively when a refractiveindices of the radial type gradient index lens element is expressed in aform of a poly-nominal. In embodiments of the present invention whichare to be described later, refractive indices of radial type gradientindex lens elements are given by the following formula (a):

n(r)=N ₀₀ +N ₁₀ r ² +N ₂₀ r ⁴+  (a)

wherein the reference symbol r represents a distance as measured from anoptical axis in a radial direction of the radial type gradient indexelement and the reference symbol n(r) designates a refractive index of aportion of the radial type gradient index lens element located at thedistance r.

Further, the objective lens system according to the present invention ischaracterized in: that it is composed, in order from the object side, ofa first lens unit having a negative refractive power, and a second lensunit having a positive refractive power, that at least the second lensunit comprises a radial type gradient index lens element having arefractive index distribution in a radial direction of the lens element,and that the second lens unit satisfies the following conditions (1) and(3):

1/V ₁₀<1/V ₀₀  (1)

0.05<φ_(2m)/φ<1.0  (3)

wherein the reference symbol φ_(2m) represents a refractive index of amedium of the radial type gradient lens element used in the second lensunit.

Further, it is desirable that the objective lens system according to thepresent invention described above satisfies the following condition (2):

−0.5<φ_(1m)/φ<−0.02

wherein the reference symbol φ_(1m) represents a refractive power of amedium of the radial type gradient index lens element used in the firstlens unit and the reference symbol φ designates a refractive power ofthe objective lens system as a whole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view illustrating a composition of aconventional objective lens system;

FIG. 2 shows a diagram schematically showing a fundamental compositionof the objective lens system according to the present invention;

FIG. 3 shows a diagram schematically showing the fundamental compositionof the objective lens system according to the present invention whereinan image surface is positioned in parallel with an optical axis;

FIGS. 4 through 15 show sectional views illustrating compositions offirst through twelfth embodiments of the objective lens system accordingto the present invention;

FIGS. 16A and 16B show views illustrating a composition of a thirteenthembodiment of the objective lens system according to the presentinvention;

FIGS. 17 through 21 show sectional views illustrating composition offourteenth through eighteenth embodiments of the objective lens systemaccording to the present invention;

FIGS. 22A, 22B and 22C show diagrams exemplifying a shape, beforeworking, of a lens element which is to be used in the objective lenssystem according to the present invention;

FIGS. 23A, 23B and 23C show diagrams exemplifying a shape, afterworking, of the lens system which is to be used in the objective lenssystem according to the present invention;

FIG. 24 is a diagram showing an example of a stop which is to bedisposed in the objective lens system according to the presentinvention;

FIG. 25 is a diagram showing another example of a stop which is to bedisposed in the objective lens system according to the presentinvention;

FIGS. 26 through 31 show sectional views illustrating compositions ofnineteenth through twenty-fourth embodiments of the objective lenssystem according to the present invention;

FIG. 32 shows a perspective view illustrating an endoscope which usesthe objective lens system according to the present invention;

FIG. 33 shows a side view of a non-flexible endoscope which uses theobjective lens system according to the present invention;

FIG. 34 shows a perspective view illustrating a video camera which usesthe objective lens system according to the present invention;

FIG. 35 shows a perspective view illustrating a portable TV telephonewhich uses the objective lens system according to the present invention;and FIG. 36 shows a perspective view illustrating a portable data inputunit which uses the objective lens system according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is desirable that an objective lens system which has a relativelywide field angle for use in endoscopes, for example, is configured asthe so-called retrofocus type composed, in order from the object side,of a first lens unit having a negative refractive power and a secondlens unit having a positive refractive power.

Further, it is desirable that endoscopes, for example, which areinserted into human bodies and pipings for observing interiors thereofuse objective lens systems having small diameters. When the lens systemaccording to the present invention is to be used as an objective lenssystem for endoscopes, it is desirable for reducing a diameter of thelens system to compose it by disposing a stop between a negative lensunit and a positive lens unit. By selecting such a composition, it ispossible to lower heights of rays passing through a first lens unit or asecond lens unit, thereby reducing the diameter of the lens system. Whenthe lens system has an asymmetrical composition, or is composed of apositive lens unit and a negative lens unit disposed on both sides of astop, however, offaxial aberrations are apt to be produced in largeamounts in the lens system as a whole, or it is difficult to favorablycorrect lateral chromatic aberration in particular. For favorablycorrecting lateral chromatic aberration, it is desirable to use a radialtype gradient index lens element which has a more excellentcharacteristic for correcting chromatic aberration than a homogenouslens element.

It is known that lateral chromatic aberration LTC produced by a thinradial type gradient index lens element is expressed by the followingformula (d):

LTC=K(φ_(s) /V ₀₀ +φ _(m) /v ₁₀)  (d)

wherein the reference symbol K represents a constant determineddependently on a height of an offaxial ray and an angle of a final axialray, the reference symbol φ_(s) designates a refractive power of asurface of the radial type gradient index lens element and the referencesymbol φ_(m) denotes a refractive power of a medium of the radial typegradient index lens element which is known to be approximated by thefollowing formula (e):

φ_(m)≈2N ₁₀ d _(G)  (e)

wherein the reference symbol d_(G) represents thickness of the radialtype gradient index lens element.

As is apparent from the formula (d), it is possible to control an amountof chromatic aberration to a desired value by varying V₁₀ in the secondterm of the formula.

Since the objective lens system according to the present invention hasan asymmetrical composition, like the model of thin lens elements shownin FIG. 2, wherein a first negative lens unit L₁ and a second positivelens unit L₂ are disposed in that order from the object side and on bothside of a stop S, each of the lens units refracts a ray RO coming froman offaxial object point, unlike a ray RA coming from an axial objectpoint, in a direction which is the same as a refracted direction(downward in FIG. 2) of an incident light bundle. Accordingly, lateralchromatic aberration is produced in a very large amount in the objectivelens system according to the present invention, thereby making itdifficult to compose the objective lens system, in order from the objectside, of a negative lens unit and a positive lens unit which consist oftwo homogenous lens elements.

For solving this problem or favorably correcting lateral chromaticaberration by adopting a radial type gradient index lens element, it isconceivable to select either of two cases: one where the radial indexlens element is used in the first negative lens unit and the other wherethe radial gradient lens element is used in the second positive lensunit.

Description will be made of a first composition of the objective lenssystem according to the present invention where a radial gradient indexelement is used in the first negative lens unit.

For allowing a radial type gradient index lens element to producechromatic aberration in an amount smaller than that of chromaticaberration produced by a homogenous lens element which has the samerefractive power as that of the radial type gradient index lens element,it is necessary from the formula (d) to satisfy the followingrelationship:

φ_(s) /V ₀₀+φ_(m) /V ₁₀<φ_(t) /V ₀₀

wherein the reference symbol φ_(t) represents a refractive power of thehomogenous lens element on an assumption that it has an Abbe's numberwhich is equal to that on the optical axis of the radial type gradientindex lens element.

Since the radial type gradient index lens element and the homogenouslens element which are compared with each other have the same refractivepower, we obtain:

φ_(t)=φ_(s)+φ_(m)

From the two formulae mentioned above, there establishes the followingrelationship:

φ_(s) /V ₀₀+φ_(m) /V10<φ_(s) /V ₀₀+φ_(m) /V ₀₀

The above-mentioned condition (1) is obtained from this formula.

For favorably correcting lateral chromatic aberration in the objectivelens system according to the present invention, it is desirable to use aradial type gradient index lens element satisfying the condition (1) asthe first lens unit which has the negative refractive power as describedabove. The condition (1) is required for allowing a radial type gradientindex lens element to produce chromatic aberration in an amount smallerthan that of chromatic aberration produced by a homogenous lens elementwhich has the same refractive power as that of the radial type gradientindex lens element. If the condition (1) is not satisfied, a radial typegradient index lens element cannot correct lateral chromatic aberrationmore favorably than a homogenous lens element which has the samerefractive power as that of the radial type gradient index lens element.

When a radial type gradient index lens element is to be used forcorrecting lateral chromatic aberration, it is necessary, as apparentfrom the formula (d), to take into sufficient consideration not only avalue of V₁₀ but also refractive powers of a surface and a medium. Forreducing an amount of lateral chromatic aberration to be produced by thefirst lens unit in the objective lens system according to the presentinvention, it is desirable from the formula (d) to meet the followingequation:

φ_(s) /V ₀₀+φ_(m) /V ₁₀≈0

By using a total refractive power of a surface and a mediumφ_(G)(=φ_(s)+φ_(m)) in this formula and developing it, we obtain:

φ_(m)≈φ_(G) ×V ₁₀/(V ₁₀ −V ₀₀)

Since the Abbe's number V₀₀ of a radial type gradient index lens elementordinarily has a value on the order of 30 to 80 on an optical axis andthe first lens unit has the negative refractive power, the condition (1)required for correcting chromatic aberration can be transformed asfollows:

φ_(G) ×V ₁₀/(V ₁₀ −V ₀₀)<0

It is desirable as understood from this formula that a medium has anegative refractive power. Taking into consideration this fact and therequirement to correct lateral chromatic aberration favorably in theobjective lens system as a whole, it is desirable that a radial typegradient index lens element which is to be used in the objective opticalsystem according to the present invention is made of a medium having arefractive power φ_(1m) satisfying the above-mentioned condition (2).

When a ratio of a refractive power of a medium of the first lens unitrelative to a refractive power of the objective lens system satisfiesthe condition (2), it is possible to correct lateral chromaticaberration favorably in the objective lens system as a whole. If theupper limit of −0.02 of the condition (2) is exceeded, a medium of theradial type gradient index lens element will have a weak refractivepower, thereby undesirably making it difficult to correct lateralchromatic aberration favorably in the objective lens system as a whole.If the lower limit of −0.5 of the condition (2) is not satisfied, incontrast, a medium will have too strong a refractive power, therebyovercorrecting lateral chromatic aberration.

The objective lens system according to the present invention which has asecond composition is composed of a first lens unit having a negativerefractive power and a second lens unit having a positive refractivepower; a radial type gradient index lens element being used in thesecond lens unit having the positive refractive power; and the radialtype gradient index lens element used in the second lens unit having arefractive index expressed by the above-mentioned formula (a) andsatisfying the following conditions (1) and (3):

1/V ₁₀<1/V ₀₀  (1)

0.05<φ_(2m)/φ<1.0  (3)

wherein the reference symbol φ_(2m) represents a refractive power of thegradient index lens element which is used in the second lens unit andthe reference symbol φ designates a refractive power of the objectivelens system as a whole.

Since lateral chromatic aberration is produced in a very large amount inthe objective lens system as a whole as described above, it is difficultto compose the lens system, in order from the object side, of twonegative and positive homogenous lens elements. When a radial typegradient index lens element is to be used in the second lens lens unitfor solving this problem or favorably correcting lateral chromaticaberration, it is desirable to configure this lens element so as tosatisfy the condition (1) mentioned above.

The condition (1) is required for allowing the radial type gradientindex lens element to produce chromatic aberration in an amount smallerthan that of chromatic aberration produced by a homogenous lens elementwhich has the same refractive power as that of the radial type gradientindex lens element.

If the condition (1) is not satisfied, the radial type gradient indexlens element cannot correct chromatic aberration more favorably than thehomogenous lens element having the same refractive power as that of theradial type gradient index lens element.

When a radial type gradient index lens element is to be used forcorrecting lateral chromatic aberration, it is necessary to take intoconsideration not only a value of V₁o but also refractive powers of asurface and a medium as described above. When lateral chromaticaberration produced by the second lens unit is to be reduced in theobjective lens system according to the present invention, the formula(d) is transformed, as in the case where a radial type gradient indexlens element is used in the first lens unit, into the following formula:

φ_(m)≈φ_(G) ×V ₁₀/(V ₁₀ −V ₀₀)

Since the second lens unit has a positive refractive power, it isdesirable that φ_(m) has a positive value. When a radial type gradientindex lens element is to be used in the second lens unit for correctinglateral chromatic aberration favorably in the objective lens system as awhole, it is desirable that the refractive power φ_(m) of a mediumsatisfies the above-mentioned condition (3).

When a ratio of a refractive power of medium relative to a refractivepower of the objective lens system as a whole satisfies the condition(3), it is possible to correct lateral chromatic aberration favorably inthe objective lens system as a whole.

If the lower limit of 0.05 of the condition (3) is not satisfied, amedium of the radial type gradient index lens element will have a weakrefractive power, thereby undesirably making it difficult to correctlateral chromatic aberration favorably in the objective lens system as awhole. If the upper limit of 1.0 of the condition (3) is exceeded, incontrast, a refractive power of medium will be too strong, therebyundesirably overcorrecting lateral chromatic aberration.

The condition (3) is required also for favorably correcting not onlychromatic aberration but also the other aberrations.

The objective optical system according to the present invention whichhas a wide field angle produces a positive Petzval's sum in the lenssystem as a whole, thereby tending to tilt an image surface toward theobject side. For favorably correcting this Petzval's sum, it isdesirable to use a radial type gradient index lens element in the secondlens unit which has a large positive Petzval's sum.

A Petzval's sum PTZ of a radial type gradient index lens element isexpressed by the following formula (f):

PTZ=φ _(s) /N ₀₀+φ_(m) /N ₀₀ ²

As is apparent from the formula (f) in which the denominator of thesecond term on the right side is squared, it is possible to configure aradial type gradient index lens element so as to have a Petzval's sumwhich is smaller than that of a homogenous lens element having the samerefractive power as that of the radial type gradient index lens element.

For reducing a Petzval's sum of the objective lens system with a radialtype gradient index lens element, it is desirable from the formula (f)that the radial type gradient index lens element satisfies the followingformula:

φ_(s) /N ₀₀+φ_(m) /N ₀₀ ²≈0

The formula shown below can be derived by using a total refractive powerof surface and medium φ_(G)(=φ₂+φ_(m)) in the above formula anddeveloping it.

φ_(m)≈φ_(G) ×N ₀₀/(N ₀₀−1)

Since refractive indices N₀₀ on an optical axis of radial gradient indexlens elements ordinarily have values larger than 1, or on the order of1.45 to 1.85, it is desirable for favorably correcting a Petzval's sumto use a radial type gradient index lens element which has a positiverefractive power of medium when it is to be used in the second lens unithaving the positive refractive power. If a radial type gradient indexlens element which has a negative refractive power of medium is used inthe second lens unit, the positive Petzval's sum will be furtherenlarged, hereby undesirably tilting an image surface toward the objectside.

For correcting the Petzval's sum in the objective lens system accordingto the present invention, it is desirable from the formula (f) that arefractive power of medium is strong to a certain degree or refractivepower of medium fm satisfies the condition (3). When a ratio of therefractive power of medium relative to a refractive power of theobjective lens system as a whole satisfies the condition (3), it ispossible to favorably correct the Petzval's sum in the objective lenssystem as a whole. If the lower limit of 0.05 of the condition (3) isnot satisfied, a refractive power of medium will be weak, thereby makingit difficult to correct the Petzval's sum favorably in the objectivelens system as a whole. If the upper limit of 1.0 of the condition (3)is exceeded, in contrast, a refractive power of medium will be toostrong and the Petzval's sum will be overcorrected, thereby undesirablytilting an image surface in a direction away from the object side.

As understood from the foregoing description, the condition (3) isrequired for favorably correcting lateral chromatic aberration andPetzval's sum at the same time in the objective lens system according tothe present invention (which has the second composition).

Further, an objective lens system for endoscopes is generally used incombination with a solid-state image pickup device such as CCD or an endsurface of the so-called image guide composed of an optical fiber bundlewhich is disposed on an image surface. When an objective lens system isto be combined not with a silver salt photographic film but with asolid-state image pickup device or an image guide, it is desirable forenhancing a light condensing efficiency to configure the objective lenssystem so as to be telecentric on the image side so that incident raysare as perpendicular as possible to an image surface. For configuringthe objective lens system according to the present invention so as tomeet this requirement, the second lens unit having the positiverefractive power must have an image side surface which has a positiverefractive power stronger than that of an object side surface thereof.When the image side surface of the second lens unit on which offaxialrays are relatively high has a strengthened refractive power in theobjective lens system according to the present invention having a widefield angle, however, this surface will produce coma in a large amountand it will be difficult to correct this coma favorably with ahomogenous lens element.

Accordingly, it was conceived to correct coma favoraly in the objectivelens system according to the present invention with a radial typegradient index lens element used in the second lens unit. For correctingthis aberration, it is desirable that the radial type gradient indexlens element has such a refractive index distribution as toprogressively lower refractive indices from the optical axis toward amarginal portion. An amount of coma to be produced by the image sidesurface of the second lens unit can be reduced by configuring the radialtype gradient index lens element to be used in the second lens unit soas to have such a refractive index distribution. Therefore, thecondition (3) serves for favorable correction of coma in addition tofavorable correction of lateral chromatic aberration. When the condition(3) is satisfied, the radial type gradient index lens element has arefractive index distribution wherein refractive indices areprogressively lowered from the optical axis toward the marginal portion,thereby being capable of favorably correcting coma. If the lower limitof 0.05 of the condition (3) is not satisfied, it will be difficult tofavorably correct coma with the radial type gradient index lens element.If the upper limit of 1.0 of the condition (3) is exceeded, in contrast,coma will undesirably be overcorrected.

Since the objective lens system according to the present invention has awide field angle and the asymmetrical composition which isnegative-positive in order from the object side, it tends to produceremarkable barrel form distortion which can hardly be correctedfavorably with two homogenous lens elements. For correcting the barrelform distortion favorably in the objective lens system according to thepresent invention, it is desirable to use, in the second lens unithaving the positive refractive power, a radial type gradient index lenselement which has a refractive index distribution wherein refractiveindices are progressively lowered from the optical axis toward themarginal portion. Distortion to be produced by the second lens unit canbe reduced by using a radial type gradient index lens element havingsuch a refractive index distribution. Accordingly, the condition (3)serves for favorable correction of distortion in addition to favorablecorrection of lateral chromatic aberration. When the contition (3) issatisfied, refractive indices are progressively lowered from the opticalaxis toward the marginal portion, thereby enabling to favorably correctdistortion. If the lower limit of 0.05 of the condition (3) is notsatisfied, it will be difficult to favorably correct distortion with aradial type gradient index lens element. If the upper limit of 1.0 ofthe condition (3) is not satisfied, in contrast, aberrations other thandistortion will undesirably be over-corrected.

The objective lens system according to the present invention which has athird composition is characterized in: that it is composed, in orderfrom the object side, of a first lens unit having a negative refractivepower and a second lens unit having a positive refractive power; that ituses at least one radial type gradient index lens element having arefractive index distribution in a radial direction from an optical axiswhich is expressed by the formula (a); and that at least one of thesurfaces of lens elements including the radial type gradient index lenselement composing the lens system is configured as an aspherical surfacewhich has such a shape as to weaken a refractive power of the lenselement having the aspherical surface from the optical axis toward themarginal portion.

Since the objective lens system according to the present invention has awide field angle and an asymmetrical composition which isnegative-positive in order from the object side, it tends to produceremarkable distortion in addition to lateral chromatic aberration.Though distortion can be corrected to a certain degree by using a radialtype gradient index lens element in the second lens unit as describedabove, it is desirable for correcting distortion more favorably toconfigure the objective optical system according to the presentinvention so as to comprise at least one surface of at least one lenselement which is configured as an aspherical surface having such a shapeas to weaken a refractive power of the lens element having theaspherical surface from the optical axis toward the marginal portion. Inthe objective lens system according to the present invention, barrelform distortion is produced by both the first lens unit having thenegative refractive power and the second lens unit having the positiverefractive power. When an aspherical surface is to be used in either ofthe lens units in the objective lens system, it is therefore desirableto configure the aspherical surface so as to have such a shape as toweaken a refractive power from the optical axis toward the marginalportion of the lens element using the aspherical surface. When thesecond lens unit uses a radial type gradient index lens element, forexample, it is desirable to use an aspherical surface in the first lensunit having the negative refractive power and configure the asphericalsurface so as to have such a shape as to weaken the negative refractivepower from the optical axis toward the marginal portion. Thoughdistortion produced by the second lens unit can be corrected to acertain degree by using a radial type gradient index lens element in thesecond lens unit as described above, distortion can be corrected morefavorably by using an aspherical surface in the first lens unit.

When a radial type gradient index lens element is used in the first lensunit, it is desirable to use, in the second lens unit having a positiverefractive power, an aspherical surface which has such a shape as toweaken the positive refractive power toward the optical axis to amarginal portion.

If an aspherical surface has such a shape as to strengthen therefractive power of a lens unit which comprises this aspherical surface,it will undesirably make barrel form distortion more remarkable in theobjective lens system as a whole.

It is needless to say that an effect similar to that described above canbe obtained by configuring a surface of a radial type gradient indexlens element as an aspherical surface.

For correcting lateral chromatic aberration in the objective lens systemaccording to the present invention which has the third composition, itis desirable to configure the radial type gradient index lens element soas to satisfy the condition (1). If the condition (1) is not satisfied,lateral chromatic aberration will undesirably be more remarkable in theobjective lens system according to the present invention.

It is needless to say that lateral chromatic aberration can be correctedmore favorably by using two radial type gradient index lens elementswhen the objective lens system according to the present invention is tobe composed of two negative and positive lens elements.

It is also needless to say that distortion can be corrected morefavorably by using an aspherical surface on each of the lens elementswhen the objective lens system is to be composed of two negative andpositive lens elements.

The objective lens system according to the present invention which has afourth composition is characterized in that it is composed, in orderfrom the object side, of a first lens unit having a negative refractivepower and a second lens unit having a positive refractive power. Thesecond lens unit comprises at least one radial type gradient index lenselement which has a refractive index distribution in a radial directionexpressed by the formula (a), or wherein refractive indices areprogressively lowered from an optical axis toward a marginal portion.The radial type gradient index lens element has a shape of a positivelens element. The radial type gradient index lens element satisfies thecondition (1).

When a radial type gradient index lens element is to be used in thesecond lens unit, it is desirable for correcting lateral chromaticaberration to configure it so as to satisfy the condition (1) asdescribed above, and it is desirable for correcting a Petzval's sum,coma or distortion that this lens element has a refractive indexdistribution wherein refractive indices are lowered from the opticalaxis toward the marginal portion. When not only correction ofaberrations but also manufacturing facility are taken intoconsideration, it is desirable that the radial type gradient index lenselement has a shape of a positive lens element.

Since the objective lens system according to the present invention has awide field angle, it is necessary that the second lens unit has a strongpositive refractive power. When a radial type gradient index lenselement is to be used in the second lens unit, it is therefore desirablethat the radial type gradient index lens element also has a stronglypositive refractive power. However, it is undesirable that the radialtype gradient index lens element has an extremely strong positiverefractive power of medium for a reason described below. As apparentfrom the formula (e), it is sufficient for strengthening a refractivepower of medium of a radial type gradient index lens element to enlargean absolute value of the refractive index distribution coefficient ofthe second order N₁₀ or thickness d_(G) of the radial type gradientindex lens element. However, an absolute value of the refractive indexdistribution coefficient of the second order can be enlarged only withina range limited by preparation of materials and enlargement of thicknessd_(G) makes it difficult to configure the objective lens systemcompactly, thereby making it unusable as an objective lens system forendoscopes, for example, which are desired to be compact. Accordingly,it is desirable not to impart an extremely strong positive refractivepower to a medium but to impart refractive powers to both the medium andsurfaces, or configure the radial type gradient index lens element so asto have a shape of a positive lens element.

The objective lens system according to the present invention which has afifth composition is characterized in that it is composed, in order fromthe object side, of a first lens unit having a negative refractive powerand a second lens unit having a positive refractive power; that it usesat least one lens element which is configured as a radial type gradientindex lens element having a refractive index distribution in the radialdirection from an optical axis expressed by the formula (a); and that alens unit comprising the radial type gradient index lens element or atleast one optical element disposed in the objective lens system has afunction to shield components having specific wavelengths.

By using a radial type gradient index lens element, it is possible toobtain an objective lens system which favorably corrects chromaticaberration in particular and has high imaging performance. An objectivelens system for endoscopes which comprises a solid-state image pickupdevice such as a CCD, for example, may use an infrared cut filter forcutting off rays in the infrared region since the image pickup devicehas high sensitivity at wavelengths in the infrared region. Further,such an optical system may use a low pass filter made, for example, ofquartz for eliminating noise components produced due to moiré.Furthermore, such an objective lens system may use a band cut filter forcutting off components having specific wavelengths since endoscopes maybe used not only for observing interiors of human bodies but also forcutting off diseased portions of patients with laser knives. It istherefore desirable that the objective lens system according to thepresent invention uses not only a radial type gradient index lenselement but also filters of the kinds mentioned above so that it canexhibit high imaging performance when it is used in optical systems forendoscopes which use, for example, solid-state image pickup devices suchas CCD's.

It is desirable that these filters are disposed at locations between alens element disposed on the image side and an image surface at whichoffaxial rays, in particular, are nearly in parallel with the opticalaxis. By disposing the filters as described above, it is possible toprepare approximately equal optical path lengths for axial and offaxialrays, thereby allowing the filters to exhibit their effects uniformlyover the entire image pickup surface.

Endoscopes which are to be inserted into human bodies must be compactand require compacter optical systems. When importance is laid oncompactness rather than optical performance, it is effective to disposefilters between the first lens unit and the second lens unit. Further,for obtaining an objective lens system which is compact and can bemanufactured at a low cost, it is effective to impart filter functionsto at least one of the lens elements including a radial type gradientindex lens element.

The objective lens system according to the present invention which has asixth embodiment is characterized in that it is composed, in order fromthe object side, of a first lens unit having a negative refractive powerand a second lens unit having a positive refractive power; that it usesat least one radial type gradient index lens element which has arefractive index distribution in a radial direction expressed by theformula (a); and that it uses a reflecting surface for at least a singlereflection disposed on the image side of the second lens unit.

Resolution of an optical system for endoscopes which uses a solid-stateimage pickup device such as a CCD, for example, can be enhanced byreducing the size of picture elements and arranging these pictureelements in a larger number at a higher density on the image pickupdevice. However, the size of a picture element can be reduced onlywithin a certain manufacturing limit and it is therefore conceivable toenhance resolution by enlarging an image pickup surface. However, it isdesirable that endoscopes which may be inserted into human bodies havesmaller diameters, or it is undesirable to enlarge image pickupsurfaces, thereby enlarging diameters of endoscopes.

For allowing solid-state image pickup devices having larger image pickupsurfaces to be used without enlarging diameters of endoscopes, theobjective lens system according to the present invention uses areflecting surface for at least one reflection on the image side of thesecond lens unit which makes it possible to dispose a solid-state imagepickup device, not in parallel with a radial direction of the endoscope,but in a position inclined with regard thereto. FIG. 3 shows aconceptional diagram of the objective lens system according to thepresent invention which has the sixth composition. In FIG. 3, thereference symbol L₁ represents the first negative lens unit, thereference symbol L₂ designates the second positive lens unit, thereference symbol RA denotes an axial ray, the reference symbol ROrepresents an offaxial ray, the reference symbol R designates thereflecting surface and the reference numeral 10 denotes the image pickupdevice disposed on an image pickup plane. The reflecting surface Rdisposed on the image side of the second lens unit makes it possible toarrange the solid-state image pickup device which has a large imagepickup surface nearly parallel to the optical axis. Though a system fora single reflection is illustrated in FIG. 3, it is possible, needlessto say, to obtain a similar effect by using a larger number ofreflecting surfaces for a plurality of reflections.

When a solid-state image pickup device having a large image pickupsurface is used, an image height can be enhanced in a lens system forobtaining a lens system having a wide field angle. However, it isdesired for an objective lens system to correct aberrations morefavorably over a range from a center to a marginal portion of an imageplane. For an objective lens system having a wide field angle inparticular, it is desired to favorably correct lateral chromaticaberration.

An objective lens system which has high imaging performance for axialand offaxial rays can therefore be obtained by using a radial gradientindex lens element which has high capability to correct chromaticaberration in particular. It is possible to obtain an objective lenssystem having higher performance by combining an objective lens systemwhich uses a radial type gradient index lens element with a reflectionsystem shown in FIG. 3 which permits enlarging an image pickup surface.

When a radial type gradient index lens element is used in the secondlens unit in each of the compositions (first through sixth compositions)of the objective lens system according to the present invention, it isdesirable for more favorable correction of lateral chromatic aberration,coma and distortion to satisfy the following condition (4):

−0.5<N _(10p) ·f ²<−0.01  (4)

wherein the reference symbol N_(10p) represents a refractive indexdistribution coefficient of the second order when a radial type gradientindex lens element is used in the second lens unit and the referencesymbol f designates a focal length of the objective lens system as awhole.

When the condition (4) is satisfied, a refractive power of medium of theradial type gradient index lens element has a sufficiently large value,thereby making it possible to correct lateral chromatic aberrationfavorably. Further, refractive indices are progressively lowered fromthe optical axis toward a marginal portion and a difference inrefractive index (An) is large between the optical axis and the marginalportion, thereby making it possible to favorably correct coma producedby an image side surface of the second lens unit and barrel formdistortion which poses a problem in the objective lens system as awhole.

If the upper limit of −0.01 of the condition (4) is exceeded, the radialtype gradient index lens element will have a weak refractive power ofmedium, thereby making it difficult to correct lateral chromaticaberration favorably in the objective lens system as a whole, and thedifference in refractive indices will be small between the optical axisand the marginal portion, thereby making it difficult to correct comaand distortion. If the lower limit of −0.5 of the condition (4) is notsatisfied, in contrast, the radial type gradient index lens element willhave a large refractive index difference An, thereby undesirably makingit difficult to prepare a material for the radial type gradient indexlens element.

When a radial type gradient index lens element is used in the first lensunit in the objective lens system according to the present inventionwhich has one of the compositions described above, it is desirable forcorrecting lateral chromatic aberration more favorably to satisfy thefollowing condition (5):

0.01<N _(10n) ·f ²<0.6  (5)

wherein the reference symbol N_(10n) represents the refractive indexdistribution coefficient of the second order of a gradient index lenselement when the radial type gradient index lens element is used in thefirst lens unit and the reference symbol f designates a focal length ofthe objective lens system as a whole.

When the condition (5) is satisfied, a refractive power of medium of theradial type gradient index lens element has a sufficiently large value,thereby making it possible to correct lateral chromatic aberrationfavorably.

If the lower limit of 0.01 of the condition (5) is not satisfied, theradial type gradient index lens element will have a weak refractivepower of medium, thereby making it difficult to correct lateralchromatic aberration favorably in the objective lens system as a whole.If the upper limit of 0.6 of the condition (5) is exceeded, the radialtype gradient index lens element will have a large refractive indexdifference Δn, thereby making it difficult to prepare a material for theradial type gradient index lens element.

When the radial type gradient index lens element is used in the firstlens unit or the second lens unit of the objective lens system accordingto the present invention, it is desirable for correcting lateralchromatic aberration more favorably in the lens system to satisfy thefollowing condition (6):

1/V ₁₀<0.01  (6)

The objective lens system which satisfies the condition (6) canfavorably correct lateral chromatic aberration. Since radial typegradient index lens elements have Abbe's numbers on the order of 30 to80 as described above, it is desirable from the formula (d), forallowing a radial type gradient index lens element to exhibit asufficient function for correcting chromatic aberration to satisfy thecondition (6). If the condition (6) is not satisfied, it willundesirably be impossible to sufficiently correct lateral chromaticaberration.

When a radial type gradient index lens element is used in the first lensunit of the objective lens system according to the present inventionwhich has any one of the first through sixth compositions, it isdesirable for correcting lateral chromatic aberration more favorably tosatisfy the following condition (7):

0.05<d _(Gn) /f _(Gn)<1.2  (7)

wherein the reference symbol d_(Gn) represents thickness of the radialtype gradient index lens element used in the first lens unit and thereference symbol f_(Gn) designates a focal length of the radial typegradient index lens element used in the first lens unit.

A radial type gradient index lens element can exhibit its effect tocorrect chromatic aberration when it has refractive power of mediumφ_(m) having a value which is large to a certain degree as describedabove. When a radial type gradient index lens element has extremelysmall thickness, for example, the refractive power of medium φ_(m) isweak as judged from the formula (e), thereby making it difficult tofavorably correct chromatic aberration. It is therefore desirable thatthe objective lens system according to the present invention satisfiesthe condition (7) when a radial type gradient index lens element is usedin the first lens unit.

If the lower limit of 0.05 of the condition (7) is not satisfied, therefractive power of medium will be weak, thereby undesirably making itdifficult to favorably correct lateral chromatic aberration. If theupper limit of 1.2 of the condition (7) is exceeded, in contrast, theradial type gradient index lens element will have a large thickness,thereby undesirably prolonging a total length of the objective lenssystem.

When a radial type gradient index lens element is used in the secondlens unit of the objective lens system according to the presentinvention which has any one of the composition described above, it isdesirable for correcting lateral chromatic aberration more favorably tosatisfy the following condition (8):

0.3<d _(Gp) /f _(Gp)<4.0  (8)

wherein the reference symbol d_(Gp) represents the thickness of theradial type gradient index lens element used in the second lens unit andthe reference symbol f_(Gp) designates a focal length of the radial typegradient index lens element used in the second lens unit.

When a radial type gradient index lens element has extremely smallthickness as described above, for example, the refractive power ofmedium fm is weak, thereby making it difficult to correct chromaticaberration favorably. When a radial type gradient index lens element isto be used in the second lens unit of the objective lens systemaccording to the present invention, the lens system is thereforeconfigured so as to satisfy the condition (8).

If the lower limit of 0.3 of the condition (8) is exceeded, a refractivepower of medium will be weak, thereby undesirably making it difficult tofavorably correct lateral chromatic aberration. If the upper limit of4.0 of the condition (8) is exceeded, a radial type gradient index lenselement will have large thickness, thereby undesirably enlarging theobjective lens system.

When the first lens unit is to be composed of a single negative lenselement in the objective lens system according to the present inventionwhich has any one of the compositions described above, it is desirablefor favorably correcting aberrations to satisfy the following condition(9):

0.1<1H/r ₂<1.7  (9)

wherein the reference symbol 1H represents an image height and thereference symbol r₂ designates a radius of curvature on an image sidesurface of the first lens unit having the negative refractive power.

If the lower limit of 0.1 of the condition (9) is not satisfied, theimage side surface of the first lens unit will have a weak refractivepower and it will be necessary to strengthen a negative refractive powerof an object side surface of the first lens unit on which offaxial raysare relatively high, whereby the object side surface will undesirablyproduce distortion, astigmatism, etc. in larger amounts. If the upperlimit of 1.7 of the condition (9) is exceeded, in contrast, the imageside surface of the first lens unit will have a strong negativerefractive power, thereby undesirably overcorrecting sphericalaberration in the objective lens system as a whole.

When the second lens unit is to be composed of a single positive lenselement in the objective lens system according to the present inventionwhich has any one of the compositions described above, it is desirablefor favorably correcting aberrations to satisfy the following condition(10):

−1.5<IH/r ₃<0.8  (10)

wherein the reference symbol 1H represents an image height and thereference symbol r₃ designates a radius of curvature on an object sidesurface of the second lens unit having the positive refractive power.

If the lower limit of −1.5 of the condition (10) is not satisfied, theobject side surface of the second lens unit will have a strong negativerefractive power, thereby undesirably overcorrecting sphericalaberration in the objective lens system as a whole. If the upper limitof 0.8 of the condition (10) is exceeded, in contrast, the object sidesurface will undesirably produce spherical aberration, etc. in largeramounts.

When the second lens unit is to be composed of a single positive lenselement in the objective lens system according to the present inventionwhich has any one of the compositions described above, it is desirablefor favorably correcting aberrations to satisfy the following condition

−1.0<IH/r ₄<−0.1  (11)

wherein the reference symbol IH represents an image height and thereference symbol r₄ designates a radius of curvature on an image sidesurface of the second lens unit having the positive refractive power.

If the lower limit of −1.0 of the condition (11) is not satisfied, theimage side surface of the second lens unit will have a strong positiverefractive power, thereby undesirably aggravating astigmatism anddistortion in the objective lens system as a whole. If the upper limitof −0.1 of the condition (11) is exceeded, in contrast, it will benecessary to strengthen a positive refractive power of the object sidesurface of the second lens unit, whereby the object side surface willundesirably produce astigmatism and distortion in large amounts.

When a radial type gradient index element is to be used in the firstlens unit in the objective lens system according to the presentinvention which has any one of the compositions described above, it isdesirable for correcting lateral chromatic aberration more favorably tosatisfy the following condition (12):

0.05<N _(10n) ·f _(Gn) ²<1.2  (12)

wherein the reference symbol f_(Gn) represents a focal length of theradial type gradient index lens element to be used in the first lensunit.

When the condition (12) is satisfied, the refractive power of medium ofthe radial type gradient index lens element will have a value which issufficiently large relative to its total refractive power and canfavorably correct lateral chromatic aberration. If the lower limit of0.05 of the condition (12) is not satisfied, the refractive power ofmedium will be weak, thereby undesirably making it difficult to correctlateral chromatic aberration favorably with the radial type gradientindex lens element. If the upper limit of 1.2 of the condition (12) isexceeded, refractive index difference Δn will be large, therebyundesirably making it difficult to prepare a material for the radialtype gradient index lens element.

When a radial type gradient index lens element is to be used in thesecond lens element in the objective lens system according to thepresent invention which has any one of the compositions described above,it is desirable for correcting lateral chromatic aberration, a Petzval'ssum, coma or distortion more favorably to satisfy the followingcondition (13):

−0.8<N _(10p) ·f _(Gp) ²<−0.05  (13)

wherein the reference symbol f_(Gp) represents a focal length of theradial type gradient index lens element to be used in the second lensunit.

When the condition (13) is satisfied, the refractive power of medium ofthe radial type gradient index lens will have a value which issufficiently large relative to its total refractive power and canfavorably correct lateral chromatic aberration. If the upper limit of−0.05 of the condition (13) is not satisfied, the refractive power ofmedium will be weak, thereby undesirably making it difficult to correctlateral chromatic aberration favorably with the radial type gradientindex lens element. If the lower limit of −0.8 of the condition (13) isnot satisfied, in contrast, refractive index difference Δn will belarge, thereby undesirably making it difficult to prepare a material forthe radial type gradient index lens element.

When a radial type gradient index lens element is to be used in thesecond lens unit in the objective lens system according to the presentinvention which has any one of the compositions described above, it isdesirable, for facilitating preparation of a material in addition tocorrection of lateral chromatic aberration, to satisfy the followingcondition (14):

0.1<φ_(2m)/φ<0.8  (14)

When the condition (14) is satisfied, the radial type gradient indexlens element will have a strong refractive power of medium and cancorrect lateral chromatic aberration favorably. If the lower limit of0.1 of the condition (14) is not reached, the radial type gradient indexlens element will have a weak refractive power of medium and can hardlycorrect lateral chromatic aberration favorably in the objective lenssystem as a whole, and a difference in refractive indices will be smallbetween a portion of the radial type gradient index lens element locatedon the optical axis and a marginal portion thereof, thereby making itdifficult to correct coma and distortion. If the upper limit of 0.8 ofthe condition (14) is exceeded, in contrast, the radial type gradientindex lens element will have a large refractive index difference An orlarge thickness, thereby undesirably making it difficult to prepare amaterial for the radial type gradient index lens element in the formercase or undesirably enhancing a manufacturing cost and prolonging atotal length of the objective lens system in the latter case.

When a radial type index lens element is to be used in the first lensunit in the objective lens system according to the present inventionwhich has any one of the compositions described above, it is desirable,for facilitating manufacturing of the radial type gradient index lenselement in addition to correction of lateral chromatic aberration, tosatisfy the following condition (15):

−0.25<φ_(1m)/φ<−0.05  (15)

When the condition (15) is satisfied, the radial type gradient indexlens element will have a strong refractive power of medium and canfavorably correct lateral chromatic aberration. If the upper limit of−0.05 of the condition (15) is exceeded, the radial type gradient indexlens element will undesirably have a weak refractive power of medium andcan hardly correct lateral chromatic aberration favorably. If the lowerlimit of −0.25 of the condition (15) is not reached, the radial typegradient index lens element will undesirably have a large refractiveindex difference Δn, whereby it can hardly be manufactured or it will bethicker and more expensive to manufacture.

When a radial type gradient index lens element is to be used in thefirst lens unit in the objective lens system according to the presentinvention which has any one of the compositions described above, it isdesirable, for facilitating manufacturing of the radial type gradientindex lens element in addition to correction of lateral chromaticaberration, to satisfy the following condition (16):

0.02<N _(10n) ·f ²<0.4  (16)

When the condition (16) is satisfied, a marginal portion of the radialtype gradient index lens element used in the first lens unit may have arefractive index higher than that of a portion thereof on the opticalaxis, whereby the radial type gradient index lens element has asufficiently strong refractive power of medium and can correct lateralchromatic aberration favorably.

If the lower limit of 0.02 of the condition (16) is not satisfied, theradial type gradient index lens element will have a weak refractivepower of medium, thereby making it difficult to correct lateralchromatic aberration favorably in the objective lens system as a whole.If the upper limit of 0.4 of the condition (16) is exceeded, the radialtype gradient index lens element will have a large refractive indexdifference Δn, thereby undesirable making it difficult to prepare amaterial therefor.

When a radial type gradient index lens element is to be used in thesecond lens unit of the objective lens system according to the presentinvention which has any one of the compositions described above, it isdesirable, for facilitating manufacturing of the radial type gradientindex lens element in addition to correction of lateral chromaticaberration, to satisfy the following condition (17):

−0.25<N _(10p) ·f ²<−0.04  (17)

When the condition (17) is satisfied, the radial type gradient indexlens element has a sufficiently strong refractive power of medium andcan favorably correct lateral chromatic aberration.

If the upper limit of −0.04 of the condition (17) is exceeded, theradial type gradient index lens element will have a weak refractivepower of medium, thereby making it difficult to correct lateralchromatic aberration favorably in the objective lens system as a whole.If the lower limit of −0.25 of the condition (17) is not satisfied, incontrast, the radial type gradient index lens element will have a largerefractive index difference Δn, thereby making it difficult to prepare amaterial therefor.

For correcting lateral chromatic aberration more favorably in theobjective lens system according to the present invention which has anyone of the compositions described above, it is desirable to configure aradial type gradient index lens element so as to satisfy the followingcondition (18):

1/V ₁₀<0  (18)

When the condition (18) is satisfied, the radial type gradient indexlens element produces chromatic aberration in a direction opposite tothat of chromatic aberration produced by a homogenous lens elementhaving a refractive power which is the same as that of the radial typegradient index lens element. That is to say, when a radial type gradientindex lens element which is used in the first lens unit or the secondlens unit of the objective lens system according to the presentinvention satisfies the condition (18), lateral chromatic aberration isovercorrected independently by the radial type gradient index lenselement, but this overcorrected chromatic aberration cancels lateralchromatic aberration produced by the other lens element, therebycorrecting chromatic aberration favorably in the objective lens systemas a whole.

When a radial type gradient index lens element is to be used in thefirst lens unit of the objective lens system according to the presentinvention which has any one of the compositions described above, it isdesirable, for facilitating manufacturing of the radial type gradientindex lens element in addition to correction of lateral chromaticaberration, to satisfy the following condition (19):

0.1<d _(Gn) /f _(Gn)<0.9  (19)

If the lower limit of 0.1 of the condition (19) is exceeded, the radialtype gradient index lens element will undesirably have a weak refractivepower of medium, thereby undesirably making it difficult to correctlateral chromatic aberration favorably. If the upper limit of 0.9 of thecondition (19) is exceeded, in contrast, the radial type gradient indexlens element will be thick, thereby undesirably increasing themanufacturing cost of the objective lens system.

When a radial type gradient index lens element is to be used in thefirst lens unit of the objective lens system according to the presentinvention which has any one of the composition described above, it isdesirable, for facilitating manufacturing of the radial type gradientindex lens element in addition to correction of lateral chromaticaberration, to satisfy the following condition (20):

0.7<d _(Gp) /f _(Gp)<2.8  (20)

When a radial type gradient index lens element has extremely smallthickness, for example, it has a weak refractive power of medium Am andcan hardly correct chromatic aberration favorably. When a radial typegradient index lens element is to be used in the second lens unit of theobjective lens system according to the present invention, it istherefore desirable to configure the radial type gradient index lenselement so as to satisfy the condition (20).

If the lower limit of 0.7 of the condition (20) is exceeded, the radialtype gradient index lens element will undesirably have a weak refractivepower of medium, thereby undesirably making it difficult to correctlateral chromatic aberration favorably. If the upper limit of 2.8 of thecondition (20) is exceeded, in contrast, the radial type gradient indexlens element will have large thickness, thereby undesirably increasingthe manufacturing cost of the objective lens system.

When the first lens unit is to be composed of a single negative lenselement in the objective lens system according to the present inventionwhich has any one of the compositions described above, it is desirablefor more favorable correction of aberrations to configure it so as tosatisfy the following condition (21):

0.25<IH/r ₂<1.4  (21)

If the lower limit of 0.25 of the condition (21) is not satisfied, animage side surface of the first lens unit will have a weak negativerefractive power and it will be necessary to strengthen a negativerefractive power of an object side surface of the first lens unit onwhich offaxial rays are relatively high, whereby the object side surfacewill undesirably produce distortion, astigmatism, etc. in large amounts.If the upper limit of 1.4 of the condition (21) is exceeded, incontrast, the image side surface of the first lens unit will have astrong negative refractive power, thereby undesirably overcorrectingspherical aberration in the objective lens system as a whole.

When the second lens unit is to be composed of a single positive lenselement in the objective lens system according to the present inventionwhich has any one of the compositions described above, it is desirablefor favorably correcting aberrations to satisfy the following condition(22):

−0.9<IH/r ₃<0.6  (22)

If the lower limit of −0.9 of the condition (22) is not reached, anobject side surface of the second lens unit will have a strong negativerefractive power, thereby undesirably overcorrecting sphericalaberration in the objective lens system as a whole. If the upper limitof 0.6 of the condition (22) is exceeded, in contrast, the object sidesurface will undesirably produce astigmatism, distortion, etc. in largeamounts.

When the second lens unit is to be composed of a single positive lenselement in the objective lens system according to the present inventionwhich has any one of the compositions described above, it is desirablefor favorable correction of aberrations to satisfy the followingcondition (23):

−0.8<IH/r ₄<−0.2  (23)

If the lower limit of −0.8 of the condition (23) is not reached, theimage side surface of the second lens unit will have a strong negativerefractive power, thereby undesirably aggravating astigmatism anddistortion in the objective lens system as a whole. If the upper limitof −0.2 of the condition (23) is exceeded, in contrast, it will benecessary to strengthen the positive refractive power of the object sidesurface of the second lens unit, whereby the object side surface willundesirably produce spherical aberrations, etc. in large amounts.

When a radial type gradient index lens element is to be used as thefirst lens unit of the objective lens system according to the presentinvention which has any one of the compositions described above, it isdesirable, for facilitating manufacturing of the radial type gradientindex lens element in addition to correction of lateral chromaticaberration, to satisfy the following condition (24):

0.1<N _(10n) ·f _(Gn) ²<0.85  (24)

When the condition (24) is satisfied, a refractive power of medium ofthe radial type gradient index lens element has a value sufficientlylarge relative to a total refractive power thereof, thereby making itpossible to correct lateral chromatic aberration favorably. If the lowerlimit of 0.1 of the condition (24) is not satisfied, the radial typegradient index lens element will undesirably have a weak refractivepower of medium and can hardly correct lateral chromatic aberrationfavorably. If the upper limit of 0.85 of the condition (24) is exceeded,in contrast, a refractive index difference Δn will undesirably be large,thereby making it difficult to prepare a material for the radial typegradient index lens element.

When a radial type gradient index lens element is to be used as thesecond lens element of the objective lens system according to thepresent invention which has any one of the compositions described above,it is desirable, for facilitating manufacturing of the radial typegradient index lens element in addition to correction of lateralchromatic aberration, to satisfy the following condition (25):

−0.3<N _(10p) ·f _(Gp) ²<−0.1  (25)

When the condition (25) is satisfied, a refractive power of medium ofthe radial type gradient index lens element has a value sufficientlylarge relative to a total refractive power thereof, whereby the radialtype gradient index lens element can correct lateral chromaticaberration favorably. If the upper limit of −0.1 of the condition (25)is exceeded, the refractive power of medium will undesirably beweakened, thereby undesirably making it difficult to correct lateralchromatic aberration with the radial type gradient index lens element.If the lower limit of −0.3 of the condition (25) is not reached, incontrast, the refractive index difference Δn will undesirably be large,thereby making it difficult to prepare a material for the radial typegradient index lens element.

Though it is desirable to satisfy the condition (18) for correctinglateral chromatic aberration more favorably in the objective lens systemaccording to the present invention which has any one of the compositionsdescribed above, lateral chromatic aberration is overcorrected when1/V₁₀ has a negative value which is too large. Therefore, it is furtherdesirable to satisfy the following condition (26):

−0.5<1/V ₁₀<0  (26)

When the condition (26) is satisfied, it is possible to correct lateralchromatic aberration favorably. If the upper limit of 0 of the condition(26) is exceeded, lateral chromatic aberration will undesirably beundercorrected. If the lower limit of −0.5 of the condition (26) is notreached, lateral chromatic aberration will undesirably beover-corrected.

From a viewpoint of reducing a manufacturing cost of the objective lenssystem according to the present invention, it is desirable that a radialtype gradient index lens element has a planar surface on one side orplanar surfaces on both sides.

The objective lens system according to the present invention which has aseventh composition uses a diffraction type optical element as anoptical element which is disposed in the lens system.

The objective lens system according to the present invention which hasthe seventh composition is composed, for example, of a first lens unithaving a negative refractive power, a stop and a second lens unit havinga positive refractive power: the first lens unit being composed of adiffractive optical element and the second lens unit being composed of arefractive optical element (lens).

Further, an objective lens system which can accomplish the object of thepresent invention can be obtained by composing a lens system of apositive lens unit and a positive lens unit: the positive lens unitdisposed on the object side being composed of a radial type gradientindex lens element and the lens unit disposed on the image side beingcomposed of a diffractive optical element.

The objective lens system which has each of the compositions describedabove is composed of a small number of optical elements, has a compactsize and exhibits favorable optical performance. It is therefore suitedfor use as an objective lens system for endoscopes, non-flexibleendoscopes and video cameras.

Accordingly, endoscopes, non-flexible endoscopes, video cameras,portable TV telephones, portable data input units, etc. which use theobjective lens system according to the present invention having thecompositions described above are included within the scope of thepresent invention.

Now, embodiments of the objective lens system according to the presentinvention will be described below:

Embodiment 1

focal length=1.07 mm, object distance=11.8 mm,

image height=0.97 mm, NA=0.0115, 2ω=113.1°

r_(l = ∞)   d₁ = 0.3000 n₁ = 1.48749 ν₁ = 70.21 r₂ = 1.2764   d₂ =1.2646 r_(3 = ∞ (stop))   d₃ = 0.5196 r₄ = −11.2580   d₄ = 1.8811 n₂(radial type gradient index lens) r_(5 = −1.6678)

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.72000, −0.87464 × 10⁻¹, 0.44719 × 10⁻², 0.37403× 10⁻² C line 1.71540, −0.88776 × 10⁻¹, 0.45390 × 10⁻², 0.37964 × 10⁻² Fline 1.73072, −0.84403 × 10⁻¹, 0.43154 × 10⁻², 0.36094 × 10⁻²

1/V₁₀=−0.050, 1/V₀₀=0.021, φ_(2m)/φ=0.353,

N_(10p) ·f ²=−0.100, IH/r₂=0.799, IH/r₃=−0.091

IH/r₄=−0.612, N_(10p) ·fG ²=−0.213, d_(Gp)/f_(G)=1.207

Embodiment 2

focal length=1.1 mm, object distance=14 mm,

image height=0.85 mm, NA=0.01, 2ω=96.4°

r_(l = ∞)   d₁ = 0.3000 n₁ = 1.51633 ν₁ = 64.18 r₂ = 0.9785   d₂ =0.3992 r_(3 = ∞ (stop))   d₃ = 0.1000 r₄ = −2.6225   d₄ = 2.4188 n₂(radial type gradient index lens) r_(5 = −1.8297)

radial type gradient index lens

N₀₀ N₁₀ N₂₀ d line 1.88300, −0.11306, 0.10113 × 10⁻² C line 1.87656,−0.11340, 0.10143 × 10⁻² F line 1.89821, −0.11227, 0.10042 × 10⁻²

1/V₁₀=−0.010, 1/V₀₀=0.025, φ_(2m)/φ=0.603,

N_(10p) ·f ²=−0.137, IH/r₂=0.869, IH/r₃=−0.324,

IH/r₄=−0.465, N_(10p)·f_(G) ²=−0.192, d_(Gp)/f_(G)=1.858

Embodiment 3

focal length=1.1 mm, object distance=11 mm,

image height=0.85 mm, NA=0.01, 2ω=103.5°

r₁ = −5.5224   d₁ = 0.3000 n₁ = 1.51633 ν₁ = 64.15 r₂ = 1.2132   d₂ =0.7397 r_(3 = ∞ (stop))   d₃ = 0.1000 r₄ = −4.0638   d₄ = 2.8548 n₂(radial type gradient index lens) r₅ = −2.5042

radial type gradient index lens

N₀₀ N₁₀ N₂₀ d line 1.88300, −0.10007, 0.17880 × 10⁻³ C line 1.87656,−0.99971 × 10⁻¹, 0.17863 × 10⁻³ F line 1.89821, −0.10030, 0.17922 × 10⁻³

1/V₁₀=0.003, 1/V₀₀=0.025, φ_(2m)/φ=0.627,

N_(10p)·f²=−0.120, IH/r₂=0.701, IH/r₃=−0.209,

IH/r₄=−0.339, N_(10p)·f_(G)=−0.221, d_(Gp)/f_(G)=1.922

Embodiment 4

focal length=1.02 mm, object distance=13 mm,

image height=1.0 mm, NA=0.012, 2ω=99.1°

r₁ = ∞   d₁ = 0.3842 n₁ = 1.74100 ν₁ = 52.65 r₂ = 1.4937 (asphericalsurface)   d₂ = 1.0970 r₃ = ∞ (stop)   d_(3 = 0.8723) r₄ = 11.2798   d₄= 1.6154 n₂ (radial type gradient index lens) r₅ = −1.7612

aspherical surface coefficients

P=1, A₄=−0.30311, A₆=0.20982

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.70000, −0.79349 × 10⁻¹, 0.34621 × 10⁻¹,−0.10102 × 10⁻¹ C line 1.69580, −0.81730 × 10⁻¹, 0.35660 × 10⁻¹,−0.10405 × 10⁻¹ F line 1.70980, −0.73795 × 10⁻¹, 0.32198 × 10⁻¹,−0.93948 × 10⁻²

1/V₁₀=−0.100, 1/V₀₀=0.020, φ_(2m)/φ=0.260,

N_(10p)·f²=−0.082, IH/r₂=0.670, IH/r₃=0.089,

IH/r₄=−0.568, N_(10p)·f_(G) ²=−0.201, d_(Gp)/f_(G)=1.015

Embodiment 5

focal length=1.1 mm, object distance=14 mm,

image height=0.85 mm, NA=0.01, 2ω=83.4°

r₁ = ∞   d₁ = 0.3000 n₁ = 1.51633 ν₁ = 64.15 r₂ = 2.9790 (asphericalsurface)   d₂ = 0.4409 r₃ = ∞ (stop)   d₃ = 0.1000 r₄ = −1.1911   d₄ =2.4102 n₂ (radial type gradient index lens) r₅ = −1.6277

aspherical surface coefficients

P=1, A₄=−0.55346, A₆=0.76608

radial type gradient index lens

N₀₀ N₁₀ N₂₀ d line 1.88300, −0.12090, 0.11838 × 10⁻¹ C line 1.87656,−0.12126, 0.11874 × 10⁻¹ F line 1.89821, −0.12005, 0.11755 × 10⁻¹

1/V₁₀=−0.010, 1/V₀₀=0.025, φ_(2m)/φ=0.641,

N_(10p)·f²=−0.146, IH/r₂=0.336, IH/r₃=−0.840,

IH/r₄=−0.614, N_(10p)·f_(G) ²=−0.173, d_(Gp)/f_(G)=2.017

Embodiment 6

focal length=1.37 mm, object distance=14 mm,

image height=0.85 mm, NA=0.01, 2ω=93.3°

r₁ = ∞   d₁ = 1.5026 n₂ (radial type gradient index lens) r₂ = 2.5886  d₂ = 0.8187 r₃ = ∞ (stop)   d₃ = 0.1000 r₄ = 3.7062   d₄ = 2.1446 n₂ =1.88300 ν₂ = 40.78 r₅ = −1.5967

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.65000, 0.21000 × 10⁻¹, 0.20240 × 10⁻¹, 0.14372× 10⁻¹ C line 1.64443, 0.21630 × 10⁻¹, 0.20847 × 10⁻¹, 0.14803 × 10⁻¹ Fline 1.66300, 0.19530 × 10⁻¹, 0.18823 × 10⁻¹, 0.13366 × 10⁻¹

1/V₁₀=−0.100, 1/V₀₀=0.029, φ_(1m)/φ=−0.086,

N_(10n)·f²=0.041, IH/r₂=0.328, IH/r₃=0.229,

IH/r₄=−0.532, N_(10n)·f_(G) ²=0.203, d_(Gn)/f_(G)=0.484

Embodiment 7

focal length=1.11 mm, object distance=14 mm,

image height=0.85 mm, NA=0.01, 2ω=104.8°

r₁ = ∞   d₁ = 0.2945 n₁ (radial type gradient index lens) r₂ = 1.0451  d₂ = 0.8845 r₃ = ∞ (stop)   d₃ = 0.1000 r₄ = 2.6882   d₄ = 2.0134 n₂ =1.88300 ν₂ = 40.78 r₅ = −1.5483

radial type gradient index lens

N₀₀ N₁₀ N₂₀ d line 1.51633, 0.26303, −0.10874 C line 1.51385, 0.26382,−0.10907 F line 1.52190, 0.26119, −0.10798

1/V₁₀=−0.010, 1/V₀₀=0.016, φ_(1m)/φ=−0.171,

N_(10n)·f²=0.322, IH/r₂=0.708, IH/r₃=0.244,

IH/r₄=−0.586, N_(10n)·fG²=0.609, d_(Gn)/f_(G)=0.194

Embodiment 8

focal length=1.13 mm, object distance=20 mm,

image height=0.8 mm, NA=0.007, 2ω=102.0°

r₁ = 5.5159   d₁ = 0.3200 n₁ (radial type gradient index lens) r₂ =1.1306   d₂ = 1.0270 r₃ = ∞ (stop)   d₃ = 0.1000 r₄ = 3.2778   d₄ =1.6805 n₂ = 1.81600 ν₂ = 46.62 r₅ = −1.3650

radial type gradient index lens

N₀₀ N₁₀ N₂₀ d line 1.55000, 0.21140, 0.20293 C line 1.54633, 0.21119,0.20273 F line 1.55856, 0.21190, 0.20340

1/V₁₀=0.003, 1/V₀₀=0.022, φ_(1m)/φ=−0.153,

N_(10n)·f²=0.269, IH/r₂=0.708, IH/r₃=0.244

IH/r₄=−0.586, N_(10n)·f_(G) ²=0.787, d_(Gn)/f_(G)=0.166

Embodiment 9

focal length=1.11 mm, object distance=14 mm,

image height=0.9 mm, NA=0.01, 2ω=98.3°

r₁ = ∞   d₁ = 1.3941 n₁ (radial type gradient index lens) r₂ = 1.7035  d₂ = 1.0045 r₃ = ∞ (stop)   d₃ = 0.1000 r₄ = 2.3343 (asphericalsurface)   d₄ = 2.7508 n₂ = 1.72916 ν₂ = 54.68 r₅ = −1.3745 (asphericalsurface)

aspherical surface coefficients

(4th surface) P=1, A₄=0.60639×10⁻¹, A₆=−0.86398 A₈=−0.85186

(5th surface) P=1, A₄=0.33872×10⁻¹, A₆=0.71755×10⁻¹, A₈=−0.27664×10⁻¹

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.70000, 0.53850 × 10⁻¹, −0.25175 × 10⁻⁴,−0.19807 × 10⁻² C line 1.69300, 0.55466 × 10⁻¹, −0.25931 × 10⁻⁴,−0.20401 × 10⁻² F line 1.71633, 0.50081 × 10⁻¹, −0.23413 × 10⁻⁴,−0.18420 × 10⁻²

1/V₁₀=−0.100, 1/V₀₀=0.033, φ_(1m)/φ=−0.167,

N_(10n)·f²=0.066, IH/r₂=0.528, IH/r₃=0.386,

IH/r₄=−0.655, N_(10n)·f_(G) ²=0.155, d_(Gn)/f_(G)=0.822

Embodiment 10

focal length=1.01 mm, object distance=11.8 mm,

image height=0.97 mm, NA=0.0115, 2ω=115.2°

r₁ = ∞   d₁ = 0.4395 n₁ (radial type gradient index lens 1) r₂ = 1.9633  d₂ = 1.1006 r₃ = ∞ (stop)   d₃ = 0.7310 r₄ = 7.6592   d₄ = 2.0532 n₂(radial type gradient index lens 2) r₅ = −1.8838

radial type gradient index lens 1

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.65000, 0.19087, −0.20292, 0.10047 C line1.64443, 0.19468, −0.20698, 0.10248 F line 1.66300, 0.18196, −0.19345,0.95778 × 10⁻¹

radial type gradient index lens 2

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.72000, −0.64751 × 10⁻¹, 0.21345 × 10⁻¹,−0.39150 × 10⁻² C line 1.71568, −0.66046 × 10⁻¹, 0.21772 × 10⁻¹,−0.39933 × 10⁻² F line 1.73008, −0.61729 × 10⁻¹, 0.20349 × 10⁻¹,−0.37323 × 10⁻²

1st lens

1/V₁₀=−0.067, 1/V₀₀=0.029, φ_(2m)/φ=0.433,

N_(10p)·f²=−0.066, IH/r₂=0.494, IH/r₃=0.127,

IH/r₄=−0.515, N_(10p)·f_(G) ²=−0.172, d_(Gp)/f_(G)=1.260

2st lens

1/V₁₀=−0.067, 1/V₀₀=0.020, φ_(1m)/φ=−0.170,

N_(10n)·f²=0.195, IH/r₂=0.494, IH/r₃=0.127,

IH/r₄=−0.515, N_(10n)·f_(G) ²=0.741, d_(Gn)/f_(G)=0.233

Embodiment 11

focal length=1.48 mm, object distance=11.6 mm,

image height=1.54 mm, NA=0.0082, 2ω=140.3°

r₁ = ∞   d₁ = 0.3999 n₁ = 1.65160 ν₁ = 58.52 r₂ = 1.3620   d₂ = 1.2404r₃ = ∞ (stop)   d₃ = 0.4587 r₄ = 27.7558   d₄ = 3.0744 n₂ (radial typegradient index lens) r₅ = −2.1899   d₅ = 0.5724 r₆ = ∞   d₆ = 0.4000 n₃= 1.51633 ν₃ = 64.15 r₇ = ∞   d₇ = 0.0300 r₈ = ∞   d₈ = 0.6200 n₄ =1.52000 ν₄ = 74.00 r₉ = ∞   d₉ = 0.0300 r₁₀ = ∞   d₁₀ = 0.4000 n₅ =1.51633 ν₅ = 64.15 r₁₁ = ∞   d₁₁ = 0.4800 r₁₂ = ∞   d₁₂ = 1.1000 n₆ =1.51633 ν₆ 64.15 r₁₃ = ∞   d₁₃ = 1.0000 n₇ = 1.51633 ν₇ = 64.15 r₁₄ = ∞

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.63300, −0.37202 × 10⁻¹, 0.26687 × 10⁻², 0.94109× 10⁻³ C line 1.62841, −0.37680 × 10⁻¹, 0.21318 × 10⁻², 0.11022 × 10⁻² Fline 1.64370, −0.36087 × 10⁻¹, 0.39213 × 10⁻², 0.56515 × 10⁻³

1/V₁₀=−0.043, 1/V₀₀=0.024, φ_(2m)/φ=0.339,

N_(10p)·f²=−0.082, IH/r₂=1.130, IH/r₃=0.055,

IH/r₄=−0.703, N_(10p)−f_(G) ²=−0.185, d_(Gp)/f_(G)=1.378

Embodiment 12

focal length=0.83 mm, object distance=9.3 mm,

image height=0.8 mm, NA=0.0115, 2ω=128.9°

r₁ = ∞   d₁ = 0.3700 n₁ = 1.51633 ν₁ = 64.15 r₂ = 1.1462   d₂ = 1.7257r₃ = ∞ (stop)   d₃ = 0.7547 r₄ = 3.0329   d₄ = 1.2159 n₂ (radial typegradient index lens) r₅ = −1.5583   d₅ = 0.0300 r₆ = ∞   d₆ = 0.4000 n₃= 1.52287 ν = 59.89 r₇ = ∞   d₇ = 0.0300 r₈ =∞   d₈ = 0.6200 n₄ =1.52000 ν₄ = 74.00 r₉ = ∞   d₉ = 0.0300 r₁₀ = ∞   d₁₀ = 0.4000 n₅ =1.51633 ν₅ = 64.15 r₁₁ = ∞   d₁₁ = 0.0300 r₁₂ = ∞   d₁₂ = 1.0000 n₆ =1.51633 ν₆ = 64.15 r₁₃ = ∞

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.63300, −0.82149 × 10⁻¹, 0.25774 × 10⁻¹, 0.17638× 10⁻¹ C line 1.62848, −0.82067 × 10⁻¹, 0.25748 × 10⁻¹, 0.17621 × 10⁻¹ Fline 1.64377, −0.82341 × 10⁻¹, 0.25834 × 10⁻¹, 0.17679 × 10⁻¹ g line1.65257, −0.82452 × 10⁻¹, 0.25869 × 10⁻¹, 0.17703 × 10⁻¹

1/V₁₀=0.003, 1/V₀₀=0.024, φ_(2m)/φ=0.166,

N_(10p)·f²=−0.057, IH/r₂=0.698, IH/r₃=0.264,

IH/r₄=−0.513, N_(10p)·f_(G) ²=−0.166, d_(Gp)/f_(G)=0.855

Embodiment 13

focal length=1.04 mm, object distance=13 mm,

image height=1.1 mm, NA=0.01, 2ω=140.5°

r₁ = ∞   d₁ = 0.3500 n₁ = 1.51633 ν₁ = 64.15 r₂ = 0.8824   d₂ = 0.8722r₃ = ∞ (stop)   d₃ = 0.1000 r₄ = ∞   d₄ = 2.4666 n₂ (radial typegradient index lens) r₅ = −1.6539

radial type gradient index lens

N₀₀ N₁₀ N₂₀ d line 1.70000, −0.82964 × 10⁻¹, 0.16995 × 10⁻¹ C line1.69475, −0.84814 × 10⁻¹, 0.17477 × 10⁻¹ F line 1.71225, −0.78645 ×10⁻¹, 0.15871 × 10⁻¹

1/V₁₀=−0.074, 1/V₀₀=0.025, φ_(2m)/φ=0.426,

N_(10p)·f²=−0.090, IH/r₂=1.245, IH/r₃=0.000

IH/r₄=−0.665, N_(10p)·f_(G) ²=−0.183, d_(Gp)/f_(G)=1.661

Embodiment 14

focal length=0.99 mm, object distance=13 mm,

image height=1.1 mm, NA=0.01, 2ω=108.9°

r₁ = ∞   d₁ = 0.3500 n₁ = 1.53996 ν₁ = 59.57 r₂ = 2.3039 (asphericalsurface)   d₂ = 0.7372 r₃ = ∞ (stop)   d₃ = 0.1000 r₄ = −1.9109   d₄ =2.4559 n₂ (radial type gradient index lens) r₅ = −1.5046

aspherical surface coefficients

P=1, A₄=−0.28151, A₆ =0.21002, A ₈=−0.73150×10⁻¹

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.70000, −0.14024, 0.31859 × 10⁻¹, −0.54102 ×10⁻² C line 1.69475, −0.14257, 0.32390 × 10⁻¹, −0.55004 × 10⁻² F line1.71225, −0.13478, 0.30620 × 10⁻¹, −0.51998 × 10⁻²

1/V₁₀=−0.056, 1/V₀₀=0.025, φ_(2m)/φ=0.681,

N_(10p)·f²=−0.137, IH/r₂=0.478, IH/r₃=−0.576,

IH/r₄=−0.731, N_(10p)·f_(G) ²=−0.198, d_(Gp)/f_(G)=2.066

Embodiment 15

focal length=1.06 mm, object distance=14 mm,

image height=0.85 mm, NA=0.01, 2ω=98.5°

r₁ = ∞   d₁ = 0.3000 n₁ = 1.51633 ν₁ = 64.15 r₂ = 1.2020   d₂ = 0.4079r₃ = ∞ (stop)   d₃ = 0.0500 r₄ = ∞   d₄ = 1.8037 n₂ (radial typegradient index lens) r₅ = −1.1988   d₅ = 0.2000 r₆ = ∞   d₆ = 2.0000 n₃= 1.51633 ν₃ = 64.15 r₇ = ∞

radial type gradient index lens

N₀₀ N₁₀ N₂₀ d line 1.75000, −0.10566, 0.60988 × 10⁻¹ C line 1.74500,−0.10598, 0.61170 × 10⁻¹ F line 1.76167, −0.10492, 0.60561 × 10⁻¹

1/V₁₀=−0.01, 1/V₀₀=0.022, φ_(2m)/φ=0.404,

N_(10p)·f²=−0.119, IH/r₂=0.707, IH/r₃=0

IH/r₄=−0.709, N_(10p)·f_(G) ²=−0.142, d_(Gp)/f_(G)=1.558

Embodiment 16

focal length=0.87 mm, object distance=11 mm,

image height=0.8 mm, NA=0.011, 2ω=131.3°

r₁ = ∞   d₁ = 0.3600 n₁ = 1.88300 ν₁ = 40.78 r₂ = 0.7400   d₂ = 0.6000r₃ = 1.6000   d₃ = 5.1250 n₂ (radial type gradient index lens) r₄ = ∞

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.70000, −0.12580, 0.78000 × 10⁻², −0.47000 ×10⁻³ C line 1.69475, −0.12567, 0.77985 × 10⁻², −0.47001 × 10⁻³ F line1.71225, −0.12609, 0.79935 × 10⁻², −0.48176 × 10⁻³

1/V₁₀=0.003, 1/V₀₀=0.025, φ_(2m)/φ=0.499,

N_(10p)·f²=−0.087, IH/r₂=1.081, IH/r₃=0.5,

IH/r₄=0, N_(10p)·f_(G) ²=−0.676, d_(Gp)/f_(G)=2.211

Embodiment 17

focal length=0.84 mm, object distance=11 mm,

image height=0.8 mm, NA=0.011, 2ω=129.2°

r₁ = ∞   d₁ = 0.3600 n₁ = 1.88300 ν₁ = 40.78 r₂ = 0.6800   d₂ = 0.8000r₃ = ∞   d₃ = 3.8000 n₂ (radial type gradient index lens) r₄ = ∞   d₄ =2.8000 n₃ = 1.51633 ν₃ = 64.15 r₅ = ∞

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.70000, −0.12580, 0.78000 × 10⁻², −0.47000 ×10⁻³ C line 1.69475, −0.12567, 0.77985 × 10⁻², −0.47001 × 10⁻³ F line1.71225, −0.12609, 0.79935 × 10⁻², −0.48176 × 10⁻³

1/V₁₀=0.003, 1/V₀₀=0.025, φ_(2m)/φ=0.549,

N_(10p)·f²=−0.090, IH/r₂=1.177, IH/r₃=0,

IH/r₄=0, N_(10p)·f_(G) ²=−0.298, d_(Gp)/f_(G)=2.471

Embodiment 18

focal length=0.96 mm, object distance=10 mm,

image height=0.85 mm, NA=0.01, 2ω=108.5°

r₁ = ∞   d₁ = 0.3500 n₁ = 1.77250 ν₁ = 49.60 r₂ = 1.3385   d₂ = 0.3000r₃ = ∞   d₃ = 2.0000 n₂ = 1.51633 ν₂ = 64.15 r₄ = ∞ (stop)   d₄ = 0.0500r₅ ∞ d₅ = 2.3091 n₃ (radical type gradient index lens) r₆ =−1.4778

radial type gradient index lens

N₀₀ N₁₀ N₂₀ d line 1.75000, −0.33160 × 10⁻¹, 0.27722 × 10⁻¹ C line1.74500, −0.33359 × 10⁻¹, 0.27888 × 10⁻¹ F line 1.76167, −0.32696 ×10⁻¹, 0.27334 × 10⁻¹

1/V₁₀=−0.02, 1/V₀₀=0.022, φ_(2m)/φ=0.147,

N_(10p)·f²=−0.031, IH/r₂=0.717, IH/r₃=0,

IH/r₄=−0.650, N_(10p)−f_(G) ²=−0.091, d_(Gp)/f_(G)=1.397

Embodiment 19

f=0.77, F/4.7, 2ω=113.3°

r₁ = ∞   d₁ = 0.3200 n₁ = 1.51633 ν₁ = 64.15 r₂ = 0.4025   d₂ = 0.2229r₃ = ∞   d₃ = 0.6556 n₂ = 1.84666 ν₂ = 23.78 r₄ = −0.9538   d₄ = 0.1230r₅ ∞ (stop)   d₅ = 0.6316 r₆ = 2.1852   d₆ = 0.8955 n₃ (radial typegradient index lens) r₇ = −3.2816   d₇ = 0.3800 r₈ = ∞   d₈ = 0.7500 n₄= 1.53172 ν₄ = 48.91 r₉ = ∞

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.65000, −1.5405 × 10^(−1,) 1.5784 × 10⁻¹, 2.9179× 10⁻² C line 1.64512, −1.5400 × 10⁻¹, 1.5784 × 10⁻¹, 2.9179 × 10⁻² Fline 1.66138, −1.5418 × 10⁻¹, 1.5784 × 10⁻¹, 2.9179 × 10⁻² g line1.67088, −1.5430 × 10⁻¹, 1.5784 × 10⁻¹, 2.9179 × 10⁻²

Embodiment 20

f=0.65, F/4.25, 2ω=113°

r₁ = ∞   d₁ = 0.3200 n₁ = 1.51633 ν₁ = 64.15 r₂ = 0.4397   d₂ = 0.1672r₃ = ∞   d₃ = 0.7894 n₃ (radial type gradient index lens) r₄ =(stop)  d₄ = 0.2910 r₅ 2.1800   d₅ = 0.5912 n₃ = 1.88300 ν₃ = 40.78 r₆=−1.2158   d₆ = 0.3800 r₇ = ∞   d₇ = 0.7500 n₄= 1.53172 ν₄ = 48.91 r₈ =∞

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.65200, −5.4253 × 10⁻¹, 6.6408 × 10⁻¹, 1.0097 Cline 1.64616, −5.3145 × 10⁻¹, 6.5052 × 10⁻¹, 9.8907 × 10⁻¹ F line1.66562, −5.6836 × 10⁻¹, 6.9570 × 10⁻¹, 1.0578 g line 1.67733, −5.9247 ×10⁻¹, 7.2632 × 10⁻¹, 1.1002

Embodiment 21

f=2.47, F/2.8, 2ω=68.1°

r₁ = 3.0226   d₁ = 1.7507 n₁ (radial type gradient index lens) r₂ =1.4700   d₂ = 1.6154 r₃ = ∞ (stop)   d₃ = 1.1572 r₄ = 3.7806   d₄ =2.4588 n₂ = 1.69680 ν₂ = 55.53 r₅ = −2.2830 (aspherical surface)   d₅ =0.7143 r₆ = ∞   d₆ = 0.7500 n₃ = 1.48749 ν₃ = 70.21 r₇ = ∞

aspherical surface coefficients

P=1, A₄=3.1652×10^(−2,) A₆=5.4023×10^(−5,) A₈=5.2354×10⁻⁴

radial type gradient index lens

N₀₀ N₁₀ N₂₀ d line 1.60000, −8.0063 × 10⁻³, −2.4555 × 10⁻³ C line1.59400, −7.7381 × 10⁻³, −2.3621 × 10⁻³ F line 1.61400, −8.5860 × 10⁻³,−2.6962 × 10⁻³ g line 1.62646, −8.8951 × 10⁻³, −2.9391 × 10⁻³

Embodiment 22

f=0.96, F/4.25, 2ω=112.9°

r₁ = ∞   d₁ = 0.3000 n₁ = 1.51633 ν₁ = 64.15 r₂ = ∞   d₂ = 0.1500 r₃ =−1.7433   d₃ = 2.3691 n₂ (radial type gradient index lens) r₄ = −0.9326  d₄ = 0.3800 r₅ = ∞   d₅ = 0.7500 n₃ = 1.53172 ν₃ = 48.91 r₆ = ∞

racial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.57000, −1.8143 × 10^(−1,) 9.8792 × 10⁻²,−1.8066 × 10⁻¹ C line 1.56715, −1.7983 × 10⁻¹, 9.7920 × 10⁻², −1.7907 ×10⁻¹ F line 1.57665, −1.8517 × 10⁻¹, 1.0083 × 10⁻¹, −1.8338 × 10⁻¹ gline 1.58176, −1.8795 × 10⁻¹, 1.0115 × 10⁻¹, −1.8621 × 10⁻¹

Embodiment 23

f=3.1, F/2.8, 2ω=65.4°

r₁ = ∞ (stop)   d₁ = 0.1000 r₂ = ∞   d₂ = 4.6740 n₁ (radial typegradient index lens) r₃ = −3.7203   d₃ = 1.9817 r₄ = 7000.0000   d₄ =0.0010 n₂ = 1000 ν₂ = −3.45 (DOE) r₅ = ∞   d₅ = 0.5000 n₃ = 1.45851 ν₃ =66.75 r₆ = ∞   d₆ = 0.7500 n₄= 1.53172 ν₄ = 48.91 r₇ = ∞

aspherical surface coefficients

(diffraction type optical element)

P = 1, A₄ = −2.1097 × 10⁻⁵, A₆ = 5.0147 × 10⁻⁶, A₈ = −3.0443 × 10⁻⁷, A₁₀= −1.3365 × 10⁻⁸

radial type gradient index lens

N₀₀ N₁₀ N₂₀ N₃₀ d line 1.70000, −1.4314 × 10⁻², −7.4587 × 10⁻⁴, 3.4977 ×10⁻⁴ C line 1.69580, −1.4176 × 10⁻², −7.4587 × 10⁻⁴, 3.4977 × 10⁻⁴ Fline 1.70980, −1.4636 × 10⁻², −7.4587 × 10⁻⁴, 3.4977 × 10⁻⁴ g line1.71757, −1.4899 × 10⁻², −7.4587 × 10⁻⁴, 3.4977 × 10⁻⁴

Embodiment 24

f=3.4, F/2.8, 2ω=60.4°

r₁ = 3.9329 d₁ = 2.1359 n₁ = 1.58423 ν₁ = 30.49 r₂ = 1.9115 d₂ = 0 n₂ =1000 ν₂ = −3.45 (DOE) r₃ = 1.9114 d₃ = 1.5050 r₄ = ∞ (stop) d₄ = 1.3716r₅ = 4.6967 d₅ = 3.3378 n₃ = 1.69680 ν₃ = 55.53 r₆ = −2.9928 (asphericalsurface) d₆ = 1.8296 r₇ = ∞ d₇ = 0.7500 n₄ = 1.48749 ν₄ = 70.21 r₈ = ∞d₈ = 1.2084 r₉ = ∞ (image)

aspherical surface coefficients

(diffraction type optical element)

P=1, A₄=9.1010×10⁻⁶, A₆=−6.4422×10⁻⁶,

(6th surface)

P = 1, A₄ = 1.1402 × 10⁻², A₆ = −9.8667 × 10⁻⁵, A₈ = 1.5019 × 10⁻⁴

wherein the reference symbols r₁, r₂, . . . represent radii of curvatureon surfaces of respective lens elements, the reference symbols d₁, d₂, .. . designate thicknesses of the respective lens elements and air-spacesreserved therebetween, the reference symbols n₁, n₂. . . denoterefractive indices of the respective lens elements, and the referencesymbols υ₁, υ₂, . . . represent Abbe's numbers of the respective lenselements.

The first embodiment of the objective lens system according to thepresent invention has a composition illustrated in FIG. 4. Speakingconcretely, it is composed of two lens units, i.e., in order from theobject side, a first lens unit consisting of a negative lens element, astop and a second lens unit consisting of a positive lens element. Thesecond lens unit having the positive refractive power is configured as aradial type gradient index lens element. The first lens unit is composedof a homogenous lens element which has a planar surface on the objectside and a concave surface on the image side, whereas the second lensunit is composed of a radial type gradient index lens element having ameniscus shape which has a concave surface on the object side.

Though it is ordinarily difficult to favorably correct lateral chromaticaberration in particular with two lens elements, the lens systempreferred as the first embodiment is capable of favorably correctinglateral chromatic aberration by using the radial type gradient indexlens element as the second lens unit.

Further, distortion and coma produced by the second lens unit arefavorably corrected by the radial type gradient index lens element usedas the second lens unit which has a refractive index distributionwherein refractive indices are progressively lowered from an opticalaxis toward a marginal portion.

In the first embodiment, lateral chromatic aberration in particular iscorrected favorably by configuring the radial type gradient index lenselement used as the second lens unit so as to satisfy the condition(18).

Furthermore, the object side planar surface of the first lens unit iseffective for lowering a cost required for polishing the lens element.

Though the first embodiment is composed only of the two lens elements,it favorably corrects aberrations and has high optical performance.

The second embodiment of the present invention is an objective lenssystem which has a composition illustrated in FIG. 5. Speakingconcretely, it is composed of two lens elements, i.e., in order from theobject side, a first lens unit composed of a negative lens element, astop and a second lens unit composed of a positive lens element. Thesecond lens unit having the positive refractive power is configured as aradial type gradient index lens element.

The second embodiment is an example in which the objective lens systemis configured so as to have a total length that is shorter than that ofthe first embodiment. In the second embodiment also, lateral chromaticaberration in particular is favorably corrected by using the radial typegradient index lens element as the second lens unit.

The radial type gradient index lens element has a meniscus shape whichhas a concave surface on the object side. When a radial type gradientindex lens element has such a meniscus shape, its refractive power ofsurface and refractive power of medium which are weaker and strongerrespectively than a refractive power of surface and a refractive powerof medium of a radial type gradient index lens element having a biconvexshape and a refractive power which is the same as that of the radialtype gradient index lens element. When a radial type gradient index lenselement has a strong refractive power of medium, its effect forcorrecting chromatic aberration can be effectively utilized as seen fromthe formula (d). For effectively utilizing the effect of a radial typegradient index lens element, the second embodiment adopts the radialtype gradient index lens element which has the meniscus shape. Themeniscus shape which has the concave surface on the object side iseffective in particular for preventing offaxial aberrations from beingaggravated.

In the second embodiment, the outer circumferential portion locatedoutside an effective diameter of the first lens unit and the second lensunit is configured as a nearly planar surfaces on which both the lensunits are cemented or kept in contact with each other. By cementingthese two lens units into an integrated part, it is possible to simplifya structure of a lens barrel and facilitate assembly of the objectivelens system.

The second embodiment also has high optical performance though it iscomposed only of the two lens elements.

The third embodiment of the present invention is an objective lenssystem which has a composition illustrated in FIG. 6. That is to say,the objective lens system is composed of two lens units, in order fromthe object side, of a first lens unit composed of a negative lenselement, a stop, and a second lens unit composed of a positive lenselement. The second lens unit having the positive refractive power isconfigured as a radial type gradient index lens element. The first lensunit is composed of a homogenous lens element having a biconcave shape,whereas the second lens unit is composed of a radial type gradient indexlens element having a meniscus shape which has a concave surface on theobject side.

Though it is ordinarily difficult to favorably correct lateral chromaticaberration with two lens elements, the objective lens system accordingto the present invention is capable of favorably correcting lateralchromatic aberration by using the radial type gradient index lenselement as the second lens unit.

The third embodiment is an example wherein amounts of aberrations to beproduced by the first lens unit are reduced by selecting the biconcavesurface for the first lens unit so that a power of this lens unit isshared between the surfaces.

Though 1/V₁₀ of the radial type gradient index lens element has apositive value, lateral chromatic aberration is favorably corrected bysatisfying the condition (1).

The objective lens system preferred as the third embodiment also hashigh optical performance though it is composed only of the two lenselements.

The fourth embodiment of the present invention is an objective lenssystem which has a composition shown in FIG. 7. Concretely, it iscomposed of two lens units, in order from the object side, a first lensunit composed of a negative lens element, a stop and a second lens unitcomposed of a positive lens element. The second lens unit having thepositive refractive power is configured as a radial type gradient indexlens element. The first lens unit has a planar surface on the objectside and a concave surface on the image side configured as an asphericalsurface which weakens a negative refractive power as portions of theaspherical surface are farther from an optical axis toward a marginalportion. The second lens unit is composed of a radial type gradientindex lens element which has a biconvex shape.

The aspherical surface used in this embodiment has a shape expressed bythe following formula:$x = {\frac{y^{2}/r}{1 + \sqrt{1 - {p\left( {y/r} \right)}^{2}}} + {\Sigma \quad A_{2i}y^{2i}}}$

wherein a direction along the optical axis is taken as the x axis, adirection perpendicular to the optical axis is taken as the y axis, thereference symbol r represents a radius of curvature on the optical axis,the reference symbol p designates a conical constant and the referencesymbol A_(2i) denotes an aspherical surface coefficient.

In the fourth embodiment, the aspherical surface used in the first lensunit is capable of favorably correcting mainly barrel form distortionproduced in the lens system as a whole.

Though the first lens unit has the planar surface on the object side inthe fourth embodiment, a similar effect can be obtained by using anaspherical surface as the object side surface of the first lens unit.

The fourth embodiment is an example wherein the second lens unit isconfigured as the biconvex radial type gradient index lens element whichhas a weakened refractive power of medium and a strengthened refractivepower of surface. Accordingly, the radial type gradient index lenselement can have a small refractive index difference An contributing toa refractive power of medium, thereby enhancing productibility orshortening a time required for imparting a refractive index distributionat a stage to prepare a material for the radial type gradient index lenselement.

In spite of the fact that the fourth embodiment is composed only of thetwo lens elements, it has high optical performance.

It is possible to obtain a similar effect by using, in place of theaspherical surface disposed in the first lens unit, an axial typegradient index lens element having a refractive index continuouslyvarying in the direction along the optical axis.

The fifth embodiment of the present invention is an objective lenssystem which has a composition illustrated in FIG. 8. That is to say, itis composed of two lens elements, in order from the object side, a firstlens unit composed of a negative lens element, a stop and a second lensunit composed of a positive lens element configured as a radial typegradient index lens element. The first lens unit has a planar surface onthe object side and a concave surface on the image side which isconfigured as an aspherical surface which has such a shape as to weakena negative refractive power as portions of the aspherical surface arefarther from the optical axis toward a marginal portion, whereas thesecond lens unit is configured as a radial type gradient index lenselement having a biconvex shape. The aspherical surface used in thefirst lens unit is capable of favorably correcting barrel formdistortion which is produced in the objective lens system as a whole asin the fourth embodiment.

For configuring compactly a tip of an endoscope which is to comprise theobjective lens system according to the present invention, an outercircumferential portion of the radial type gradient index lens elementused as the second lens unit is cut off as shown in FIG. 8. A tip of theobjective lens system can be made thinner since a lens barrel or thelike can be disposed, for example, as the slashed location 3 on the cutportion 2.

The fifth embodiment is an example wherein the objective lens system hasa total length that is shorter than that of the fourth embodiment.Though the fifth embodiment is composed only of the two lens elements,it has high optical performance.

The sixth embodiment of the present invention has a composition shown inFIG. 9. Speaking concretely, it is composed of two lens units, in orderfrom the object side, a first lens unit composed of a negative lenselement, a stop and a second lens unit composed of a positive lenselement. The first lens unit having the negative refractive power isconfigured as a radial type gradient index lens element. The first lensunit is a radial type gradient index lens element which has a planarsurface on the object side and a concave surface on the image side,whereas the second lens unit is a biconvex homogenous lens element.

Unlike the first through fifth embodiments, the six- th embodiment usesthe radial type gradient index lens element as the first lens unit forfavorably correcting lateral chromatic aberration which poses a problemin particular in the objective lens system according to the presentinvention.

The planar surface adopted as the object side surface of the first lensunit makes it possible to reduce the cost for polishing the lenselement.

Further, aberrations are reduced by selecting the biconvex shape for thesecond lens unit so that its power is shared between the two surfaces.

Furthermore, the image side surface of the second lens unit has arefractive power of surface which is stronger than that of the objectside surface thereof so that offaxial rays are incident on an imagesurface nearly at telecentric angles.

The sixth embodiment also has high optical performance though it iscomposed only of the two lens elements.

The seventh embodiment of the present invention has a compositionillustrated in FIG. 10. That is to say, an objective lens systempreferred as the seventh embodiment is composed of two lens units, inorder from the object side, a first lens unit composed of a negativelens element, stop and a second lens unit composed of a positive lenselement. The first lens unit having the positive refractive power isconfigured as a radial type gradient index lens element. The first lensunit is a radial type gradient index lens element which has a planarsurface on the object side and a concave surface on the image side,whereas the second lens unit is a homogenous lens element which has abiconvex shape.

This embodiment also has high optical performance though it is composedonly of the two lens elements.

The eighth embodiment of the present invention is an objective lenssystem which has a composition shown in FIG. 11. This lens system iscomposed of two lens units, in order from the object side, a first lensunit composed of a negative lens element, a stop and a second lens unitcomposed of a positive lens element. The first lens unit having thepositive refractive power is configured as a radial type gradient indexlens element. The first lens unit is a radial type gradient index lenselement having a meniscus shape which has a concave surface on the imageside, whereas the second lens unit is a homogenous lens element whichhas a biconvex shape.

The composition of the eighth embodiment, in which the radial typegradient index lens element has the meniscus shape which has a concavesurface on the image side is advantageous for correcting offaxialaberrations in particular.

The eighth embodiment also has high optical performance in spite of thefact that it is composed only of the two lens elements.

The ninth embodiment of the present invention is an objective lenssystem which has a composition illustrated in FIG. 12. Speakingconcretely, this lens system is composed of two lens elements, in orderfrom the object side, a first lens unit composed of a negative lensunits, a stop and a second lens unit composed of a positive lenselement. The first lens unit having the negative refractive power isconfigured as a radial type gradient index lens element. The first lensunit is a radial type gradient index lens element having a meniscusshape which has a planar surface on the object side and a concavesurface on the image side, whereas the second lens unit has asphericalsurfaces on both sides.

Owing to a fact that an image side surface of the second lens unit onwhich offaxial rays are relatively high is configured as an asphericalsurface having such a shape as to weaken a positive refractive power asportions of the aspherical surface are farther from an optical axistoward a marginal portion, the ninth embodiment is capable of favorablycorrecting barrel form distortion produced in the lens system as awhole.

Though the ninth embodiment uses the aspherical surfaces on both sidesof the second lens unit, a similar effect can be obtained by using anaspherical surface only on one side.

The ninth embodiment also exhibits high optical performance though itconsists only of the two lens elements.

Further, a similar effect can be obtained by adopting, in place of theaspherical surfaces used in the second lens unit, an axial type gradientindex lens element which continuously varies a refractive index in adirection along the optical axis.

The tenth embodiment of the present invention is an objective lenssystem which has a composition shown in FIG. 13. This lens systemconsists of two lens units, in order from the object side, a first lensunit composed of a negative lens element, a stop and a second lens unitcomposed of a positive lens element. The first lens unit having thenegative refractive power and the second lens unit having the positiverefractive power are configured as radial type gradient index lenselements. The first lens unit is a radial type gradient index lenselement which has a planar surface on the object side and a concavesurface on the image side, whereas the second lens unit is a radial typegradient index lens element which has a bi-convex shape.

The tenth embodiment is an example wherein lateral chromatic aberrationproduced in the lens system as a whole is favorably corrected by usingradial type gradient index lens elements in both the lens units. Inother words, lateral chromatic aberration is favorably corrected by con-figuring the radial type gradient index lens element used in the firstlens unit to satisfy the condition (1) and the condition (2), anddesigning the radial type gradient index lens element used in the secondembodiment to satisfy the condition (1) and the condition (3).

Though the tenth embodiment consists only of the two lens elements, ithas high optical performance.

Preferred as an eleventh embodiment of the present invention is anobjective lens system which has a composition illustrated in FIG. 14.This lens system comprises two lens units, or is composed, in order fromthe object side, a first lens unit composed of a negative lens element,a stop, a second lens unit composed of a positive lens element andfilters F disposed on the image side of the second positive lens unit.The first lens unit is configured as a homogenous lens element which hasa planar surface on the object side and a concave surface on the imageside, whereas the second lens unit is configured as a biconcave radialtype gradient index lens element which has an object side surface havinga weak positive refractive power.

When the objective lens system according to the present invention isused in a video scope or the like which uses a solid-state image pickupdevice such as a CCD, a low pass filter and an infrared cut filtercomposed, for example, of quartz or diffraction gratings may be disposedon the object side of an image surface as exemplified in FIG. 14. Theeleventh embodiment is an example comprising such filters. In theobjective lens system according to the present invention, theabove-mentioned filters can be disposed between the negative lenselement and the positive lens element.

Preferred as the twelfth embodiment of the present invention is anobjective lens system which has a composition shown in FIG. 15. Like theeleventh embodiment, the twelfth embodiment comprises two lens units, oris composed, in order from the object side, of a first lens unitcomposed of a negative lens element, a stop, a second lens unit composedof a positive lens element, and filters which are disposed between thetwo lens elements and on the image side of the positive lens element.The first lens unit is configured as a homogenous lens element which hasa planar surface on the object side and a concave surface on the imageside, whereas the second lens unit is configured as a radial typegradient index lens element which has a biconvex shape.

The thirteenth embodiment of the present invention has a compositionillustrated in FIGS. 16A and 16B: FIG. 16A being a sectional viewshowing the thirteenth embodiment as a whole and FIG. 16B being adiagram illustrating only a portion B on an enlarged scale. Thethirteenth embodiment is composed, in order from the object side, of afirst lens unit composed of a negative lens element, a stop and a secondlens unit composed of a positive lens element. A radial type gradientindex lens element is used as the second lens unit. The first lens unithas a planar surface on the object side and a concave surface on theimage side, whereas the second lens unit has a planar surface on theobject side and a convex surface on the image side.

The thirteen embodiment is an example wherein aberrations are favorablycorrected by using a radial type gradient index lens element and a largenumber of planar surfaces are adopted for reducing a cost required forpolishing lens elements, or the object side surface of the first lensunit and the object side surface of the second lens unit are configuredto be planar.

Further, for preventing eccentricities of lens elements, in directionsalong an optical axis and perpendicular thereto in particular, whichpose a problem at a stage of assembly, an outer circumferential portion4 is obliquely chamfered as shown in FIG. 16B so that a chamferedportion 4 is limited at a location 6 of a lens barrel 5. Since a radialtype gradient index lens element which has a refractive power of mediumrequires a manufacturing precis-on stricter than that for a homogenouslens element, the thirteenth embodiment provides an effective assemblingconvenience. A similar effect can, needless to say, be obtained byassembling the first lens unit having the negative refractive power in asimilar manner.

Though the thirteenth embodiment uses a large number of planar surfaces,it has high optical performance.

The fourteenth embodiment of the present invention is an objective lenssystem which has a composition illustrated in FIG. 17. That is to say,the fourteenth embodiment is composed, in order from the object side, ofa first lens unit consisting of a negative lens element, a stop and asecond lens unit consisting of a positive lens element which isconfigured as a radial type gradient index lens element. The first lensunit has a planar surface on the object side and a concave surface onthe image side, whereas the second lens unit has a meniscus shape whichhas a concave surface on the object side.

When a radial type gradient index lens element is used as the secondlens unit, it is desirable for correcting aberrations such as sphericalaberration that the term of the fourth order N₂₀ of a refractive indexdistribution has a positive value.

The fifteenth embodiment o- the present invention has a compositionshown in FIG. 18. Speaking concretely, the fifteenth embodiment iscomposed, in order from the object side, of a first lens unit consistingof a negative lens element, a second lens unit consisting of a positivelens element and a reflecting optical element. A radial type gradientindex lens element is used as the second lens unit. In this embodiment,a light bundle emerging from the second lens unit is reflected by areflecting surface 7 of a reflecting optical element P made, forexample, of a mirror or a prism so that it is imaged onto a solid-stateimage pickup device 10 which is disposed nearly parallel to the opticalaxis. This reflecting surface makes it possible to dispose a solid-stateimage pickup device such as a CCD that is not parallel to a radialdirection of the lens elements but oblique. Though a solid-state imagepickup device 10 is disposed nearly parallel to the optical axis in thefifteenth embodiment, the image pickup device can be disposed moreobliquely by selecting an adequate inclination angle for the reflectingsurface 7 relative to the optical axis. Though the fifteenth embodimentis configured to reflect the light bundle only once, it is possible toreflect the light bundle twice, three times or more times by adopting alarger number of reflecting surfaces.

The sixteenth embodiment of the present invention is an objective lenssystem which has a composition shown in FIG. 19. That is to say, thesixteenth embodiment is composed, in order from the object side, of afirst lens unit composed of a negative lens element, a second lens unitcomposed of a positive lens element and a solid-state image pickupdevice. A radial type gradient index lens element is used as the secondlens unit. In this embodiment, the radial type gradient index lenselement has a nearly planar image side surface which is cemented or keptin close contact to or with a solid-state image pickup device 10 such asa CCD. Further, disposed in the second lens unit, is a member whichextends from an outer circumference toward an optical axis for shieldinga light bundle passing through the lens unit or functioning as anaperture stop of the objective lens system. Such a shielding member canbe constituted, for example, by forming a cut 8 from the outercircumference toward the optical axis. For preventing the cut fromproducing flare, it is desirable to color the cut 8. Further, it ispossible to cut the second lens unit along a dashed line 9 shown in FIG.25 into a front portion La and a rear portion Lb, attach a stop S to acut surface of the front portion La and cementing the two portions toeach other or bringing them into close contact with each other. Asimilar effect can be obtained by attaching the stop S to the rearportion Lb in place of the front portion La. As means for attaching thestop, it is conceivable to utilize deposition, printing or bonding astop plate. This means can be utilized for manufacturing not only anaperture stop but also a flare stop.

In the sixteenth embodiment, the aperture stop is disposed at a location1 mm apart from an object side sur- face of the second lens unit towardan image surface.

It is needless to say that an aperture stop, a flare stop or the likecan be manufactured by the means described above in each of theembodiments.

The seventeenth embodiment of the present invention has a compositionillustrated in FIG. 20. That is to say, the seventeenth embodiment iscomposed, in order from the object side, of a first lens unit composedof a negative lens element, a second lens unit composed of a positivelens element, an optical filter F and a solid-state image pickup device10. A radial type gradient index lens element is used as the second lensunit. In the seventeenth embodiment, the second lens unit has planarsurfaces on both sides for reducing the cost for polishing the lensunit. The seventeenth embodiment is an example wherein the second lensunit is cemented to the optical filter F for simplifying a structure ofa lens barrel and assembly of the objective lens system. Further, theoptical filter F is cemented or kept in close contact to or with thesolid- state image pickup device 10. Furthermore, the first lens unitand the second lens unit are cemented or kept in close contact to orwith each other on outer circumferences thereof outside an effectivediameter thereof.

In the seventeenth embodiment, an aperture stop is disposed at alocation 1 mm apart from an object side surface of the second lens unittoward an image surface.

The eighteenth embodiment of the present invention has a compositionshown in FIG. 21. That is to say, the eighteenth embodiment is composed,in order from the object side, of a first lens unit composed of anegative lens element, a regularly reflecting optical element and asecond lens unit composed of a positive lens element. The second lensunit which is disposed on the image side is configured as a radial typegradient index lens element.

Certain endoscopes are specified for oblique observation in directionsinclined relative to the optical axis. When the objective lens systemaccording to the present invention is used in an endoscope for obliqueobservation, a reflecting optical element P composed of a mirror or aprism is disposed between the first lens unit and the second lens unitas shown in FIG. 21 so that a light bundle emerging from the first lensunit is reflected by a reflecting surface 7 of the reflecting opticalelement and led to the second lens unit.

Though a lens element ordinarily has a circular sectional shape, it mayhave a different sectional shape. FIGS. 22A, 22B, 22C, 23A, 23B and 23Cexemplify lens elements which have external shapes cut or worked inaccordance with shapes of image pickup devices for configuring objectivelens systems compactly. FIGS. 22A, 22B and 22C schematically show thesecond lens unit which is used in the first through eighteenthembodiments before being shaped. FIG. 22A slows a sectional view takenin a direction from an object, FIG. 22B is a sectional view taken in adirection in parallel with the optical axis, and FIG. 22C illustrates animage circle 11 of the objective lens system and an image pickup surface12 of an image pickup device. FIGS. 23A, 23B and 23C schematically showthe lens element illustrated in FIGS. 22A, 22B and 22C in anothercondition after it is shaped. Since a solid-state image pickup devicesuch as a CCD may have a rectangular shape and a lens element ordinarilyhas a circular shape as shown in FIG. 22A, there is produced, as slashedin FIG. 22C, a region which serves for image formation but is not usedfor image pickup. Since it is desired to configure an objective lenssystem for endoscopes compacter, it is conceivable to cut off the regionof the lens element which is not used for image pickup. The eithteenthembodiment is an example wherein lens portions through which rays to becondensed outside an image pickup surface of an image pickup device arecut off as exemplified by FIGS. 23A and 23B so that an image formationrange of the lens element is nearly coincident with the image pickupsurface of the image pickup device. It is possible to obtain a compacterobjective lens system or endoscopes by configuring a lens element so asto occupy a narrower space.

Though the second lens unit is worked for narrowing a lens space in theexample described above, it is needless to say that a similar effect canbe obtained by working other lens element or optical element, forexample, the first lens unit or the reflecting optical element.

The nineteenth embodiment has a composition illustrated in FIG. 26.Speaking concretely the nineteenth embodiment is a retrofocus typeobjective lens system which is composed, in order from the object side,of a first negative lens unit, a second positive lens unit and a thirdpositive lens unit. A stop is disposed between the second lens unit andthe third lens unit. The nineteenth embodiment is an objective lenssystem which has a wide field angle and is usable for endoscopes, videocameras, etc. Though the nineteenth embodiment has a wide field anglewhich makes it rather hard to correct lateral chromatic aberration, itcorrects lateral chromatic aberration with a radial type gradient indexlens element. In the nineteenth embodiment, the first lens unit has aplano- concave shape and a function to widen a field angle, the secondlens unit has a plano-convex shape and a function to correct mainlylateral chromatic aberration produced by the first lens unit, and thethird lens unit has a biconvex shape and a main imaging function, and isconfigured as a radial type gradient index lens element.

Though it is rather hard in the nineteenth embodiment to correct comawhich is produced by the third lens unit disposed on the image side of astop, coma is corrected by configuring the third lens unit as a radialtype gradient index lens element which has such a characteristic as toprogressively lower a refractive index in a radial direction from anoptical axis. In other words, coma is corrected by imparting a positiverefractive power to a medium of the radial type gradient index lenselement. The composition described above is capable of correcting notonly coma but also distortion.

For correcting coma with a medium of a radial type gradient index lenselement, it is desirable that terms of high orders of a refractive indexdistribution satisfy the following condition (27): $\begin{matrix}{{- 0.05} < {\sum\limits_{i = 2}^{n}\quad {N_{iod}e^{2i}}} < 0.2} & (27)\end{matrix}$

wherein the reference symbol e represents an effective diameter of thelens element.

If the lower limit of −0.05 of the condition (27) is not reached, comawill be undercorrected. If the upper limit of 0.2 of the condition (27)is exceeded, in contrast, coma will be overcorrected.

It is desirable for more favorable correction of coma to satisfy thefollowing condition (28): $\begin{matrix}{0 < {\sum\limits_{i = 2}^{n}\quad {N_{iod}e^{2i}}} < 0.1} & (28)\end{matrix}$

If the lower limit of 0 of the condition (28) is not reached, coma willbe undercorrected. If the upper limit of 0.1 of the condition (28) isexceeded, in contrast, coma will be overcorrected. Either of these casesis undesirable for more favorable correction of coma.

Further, a planar surface used as an object side surface of the lenselement disposed on the image side in the nineteenth embodiment has aneffect for preventing adhesion of foreign matter such as dust.

In this embodiment, at least one lens element can have a function to cutoff components having specific wave- lengths, or a radial type gradientindex lens element can have such a function.

When an image pickup device such as a CCD which has high sensitivity inthe infrared wavelength region is to be disposed on an image pickupsurface, for example, it is desirable to provide a function to cut offcomponents having wavelengths in the infrared region. Accordingly, it isdesirable that at least one lens element has a function to cut offcomponents having specific wavelengths such as those in the infraredregion.

The function to cut off components having specific wavelengths can beobtained by forming, on a flat portion of a lens element or a flatplate, an interference film which cuts off the components havingspecific wavelengths.

A stop can be manufactured from a thin plate.

Further, amounts of aberrations produced can be reduced by configuring aradial type gradient index lens element so as to have a biconvex shape,thereby sharing a refractive power between the two surfaces.

Use of a radial type gradient index lens element makes it possible toobtain an optical system which is compact and composed of a small numberof lens elements, and has a wide field angle and favorably correctedaberrations such as chromatic aberration and coma. Accordingly, it iseffective to use an optical system such as an optical system for imageinput units for portable TV telephones and portable data input unitssuch as those shown in FIGS. 35 and 36.

The twentieth embodiment is an optical system which has a compositionillustrated in FIG. 27. It is a retro-focus type optical system which iscomposed, in order from the object side, of a first lens unit having anegative refractive power, a second lens unit having a positiverefractive power and a third lens unit having a positive refractivepower. A stop is disposed between the second lens unit and the thirdlens unit.

The twentieth embodiment is an optical system which has a wide fieldangle and is usable as an objective lens system for endoscopes or a lenssystem for video cameras, etc. Though the twentieth embodiment also hasa wide field angle and hardly allows correction of Lateral chromaticaberration in particular, aberrations Ere favorably corrected in thisembodiment by using a radial type gradient index lens element.

In the twentieth embodiment, the first lens unit is a plano-concave lenselement having a function to widen a field angle, the second lens unitis a radial type gradient index lens element having plarar surfaces onboth sides and a function to correct lateral chromatic aberrationproduced by the first lens unit, end the third lens unit has a biconvexshape and a main imaging function. The twentieth embodiment favorablycorrects lateral chromatic aberration in particular by using, as thesecond lens unit disposed on the object side of the stop, a radial typegradient index lens element made of a medium which has a positiverefractive power and a relatively strong dispersing power. The mediumhaving the positive refractive power favorably corrects a Petzval's sum.

For correcting lateral chromatic aberration by disposing a radial typegradient index lens element on a stop, it is desirable to satisfy thefollowing condition (29)

0.01<1/V₁₀<0.5  (29)

If the lower limit of 0.01 of the condition (29) is not reached, lateralchromatic aberration will be undercorrected. If the upper limit of 0.5of the condition (29) is not satisfied, in contrast, lateral chromaticaberration will be overcorrected.

For correcting lateral chromatic aberration more favorably by disposinga radial type gradient index lens element on the object side of thestop, it is desirable to satisfy the following condition (30):

0.015<1/V₁₀<0.1  (30)

If the lower limit of 0.015 of the condition (30) is not reached,lateral chromatic aberration will be undercorrected. If the upper limitof 0.1 of the condition (30) is exceeded, lateral chromatic aberrationwill be overcorrected. Either of these cases is undesirable forcorrecting lateral chromatic aberration extremely favorably.

When preparation of a material for a radial type gradient index lenselement is taken into consideration, it is desirable that 1/V₁₀ has avalue not exceeding 0.05. From viewpoints of manufacturing convenienceand cost, it is desirable to configure a radial type gradient index lenselement so as to have planar surfaces on both sides. When durability ofa radial type gradient index lens element is taken into consideration,it is desirable to make it entirely of a glass material.

The twenty-first embodiment has a composition illustrated in FIG. 28, oris a retrofocus type optical system which is composed, in order from theobject side, of a first lens unit having a negative refractive power anda second lens unit having a positive refractive power. A stop isdisposed between the first lens unit and the second lens unit. Thetwenty-first embodiment which is an optical system having a wide fieldangle is usable as an objective lens system for endoscopes or videocameras and so on. Though this optical system also has a wide fieldangle and hardly allows correction of lateral chromatic aberration inparticular, lateral chromatic aberration is favorably corrected by usinga radial type gradient index lens element.

In the optical system preferred as the twenty-first embodiment, thefirst lens unit has a negative meniscus shape which has a concavesurface on the image side, and the second lens unit has a biconvex shapeand a main imaging function. A radial type gradient index lens elementis used as the first lens unit.

Offaxial aberrations can be corrected favorably by configuring theradial type gradient index lens element used as the first lens unit soas to have the meniscus shape which has the concave surface on the sideof the stop. Lateral chromatic aberration is favorably corrected inparticular owing to a fact that the radial type gradient index lenselement has a positive refractive power of medium and is configured soas to satisfy the condition (29).

Further, coma is corrected favorably by using an aspherical surface onthe lens element disposed on the object side of the stop. Thisaspherical surface has such a shape as to weaken a positive refractivepower as portions of the aspherical surface are farther from an opticalaxis toward a marginal portion. Owing to this aspherical surface, thetwenty-first embodiment is configured as an optical system which iscompact and composed of a small number of lens elements, has a widefield angle and favorably corrects aberrations such as coma. It istherefore effective to apply the twenty-first embodiment as an opticalsystem for image intake devices for portable TV telephones and portabledate input units.

The twenty-second embodiment has a composition shown in FIG. 29, or isan optical system composed of a single radial type gradient index lenselement. The radial type gradient index lens element has a meniscusshape which has a concave surface on the object side and a positivepower of medium. A stop is disposed at a location 0.9471 mm apart froman object side surface of the lens element toward the image side.

The twenty-second embodiment has a wide field angle, and is usable as anobjective lens system for endoscopes or a lens system for video camerasand so on. Though the twenty-second embodiment which has the wide fieldangle hardly allows correction of chromatic aberration, it is correctedwith the radial type gradient index lens element. A flat glass plate isdisposed on the object side as a cover glass plate. A stop is disposedin the radial type gradient index lens element for obtaining highlysymmetrical refractive power of medium, thereby favorably correctingoffaxial aberrations. The radial type gradient index lens elementsatisfies the condition (29) for correcting lateral chromatic aberrationproduced by an object side concave or convex surface with a mediumlocated on the object side of the stop.

For reducing chromatic aberration to be produced by surfaces of theradial type gradient index lens element, it is desirable that V₀₀ has avalue of at least 30, or more desirably, at least 40. For reducingaberrations to be produced by the surfaces of the radial type gradientindex lens element, it is desirable that N₀₀ has a value of at least1.55, or more desirably, at least 1.6.

The twenty-third embodiment has a composition shown in FIG. 30, or iscomposed of two lens units, in order from the object side, a firstpositive lens unit and a second positive lens unit. The twenty-thirdembodiment is an example wherein aberrations are corrected favorably bycomposing an optical system of a radial type gradient index lens elementand a diffraction type optical element(DOE). Speaking more concretely,the twenty-third embodiment is composed, in order from the object side,of a first lens unit composed of a radial type gradient index lenselement and a second lens unit composed of a diffraction type opticalelement.

A diffraction type optical element is equivalent to a lens element whichhas a very high imaginary refractive index as described in literatureSPIE, Vol. 126, P46 (1997). For this reason, the diffractive opticalelement is assumed to be a lens element which is optically equivalent tothe optical element including aberrations to be produced thereby, andradii of curvature, thickness, a refractive index, an Abbe's number andaspherical surface coefficients of the lens element are described in thenumerical data of the twenty-third embodiment. In the twenty-thirdembodiment, a stop is disposed on the object side of the optical system(the first lens unit).

In the twenty-third embodiment, the first lens unit composed of theradial type gradient index lens element has a main imaging function andthe second lens unit composed of the diffraction type optical elementfavorably corrects lateral chromatic aberration. The twenty-thirdembodiment is an example of an optical system which is composed of asmall number of optical elements and has a wide field angle, andfavorably corrects lateral chromatic aberration with a diffractiveoptical element disposed on the image side of a stop. The diffractiontype optical element has a positive refractive power, is disposed on theimage side of the radial type gradient index lens element and has afunction to favorably correct lateral chromatic aberration produced by aconvex surface of the radial type gradient index lens element. Further,the radial type gradient index lens element has another function tofavorably correct a Petzval's sum.

The twenty-third embodiment is usable, as described above, as anobjective lens system for endoscopes or an optical system for videocameras and so on. Since the twenty-third embodiment is an opticalsystem which is compact and is composed of a small number of lenselements, and favorably corrects aberrations such as chromaticaberration owing to the fact that it uses the diffraction type opticalelement, it is effective to apply it as an optical system for imageintake devices for portable TV telephones and portable data input units.

The twenty-fourth embodiment has a composition illustrated in FIG. 31,or is composed, in order from the object side, of a first lens unithaving a negative refractive power and a second lens unit having apositive refractive power. A stop is disposed between the first lensunit and the second lens unit. In the twenty-fourth embodiment, thefirst lens unit having the negative refractive power serves mainly forwidening a field angle and the second lens unit having the positiverefractive power functions mainly for imaging.

The twenty-fourth embodiment favorably corrects aberrations by using thediffraction type optical element in place of the radial type gradientindex lens element adopted for the twenty-first embodiment.

The twenty-fourth embodiment hardly allows correction of offaxialaberrations since it has an asymmetrical refractive power distributionwherein a negative refractive power is disposed on the object side ofthe stop and a positive refractive power is disposed on the image sideof the stop. For this reason, lateral chromatic aberration is correctedfavorably by disposing the diffraction type optical element on theobject side of the stop and imparting a negative refractive power tothis optical element. Coma and distortion in particular can be correctedfavorably by using an aspherical surface on a positive lens unitdisposed on the image side of the stop and configuring this asphericalsurface so as to have such a shape as to weaken a positive refractivepower in a radial direction from an optical axis. The negative lens unithas a meniscus shape which has a concave surface on the side of the stopfor reducing amounts of offaxial aberrations in particular. Further, thepositive lens unit has a biconvex shape so that amounts of aberrationsare reduced by sharing a refractive power between two surfaces.Furthermore, the diffraction type optical element is disposed on asurface which is concave toward the stop so that differences betweenangles of incidence of paraxial rays and offaxial rays are reduced,thereby enhancing a diffraction efficiency.

FIG. 32 is a perspective view illustrating an endoscope according to thepresent invention which uses an objective lens system such as theembodiment described above. The reference numeral 21 represents theobjective lens system.

FIG. 33 is a side view of a non-flexible endoscope according to thepresent invention which uses an objective lens system such as theembodiment described above. The reference numeral 21 represents theobjective lens system.

FIG. 34 shows a video camera according to the present invention whichuses an objective lens system such as the embodiment described above.The reference numeral 21 represents the objective lens system.

FIG. 35 shows a perspective view illustrating a portable TV telephoneaccording to the present invention which uses an objective lens systemsuch as the embodiment described above. The reference numeral 21represents the objective lens system, the reference numeral 22designates an antenna for transmission and reception, the referencenumeral 23 denotes a display, and the reference numeral 24 representsswitches.

Further, FIG. 36 shows a perspective view illustrating a portable datainput unit according to the present invention which uses an objectivelens system such as the embodiment described above. The referencenumeral 21 represents the objective lens system, the reference numeral22 designates an antenna for transmission and reception, the referencenumeral 23 denotes a display, and the reference numeral 24 representsswitches.

As understood from the foregoing description, the objective lens systemfor endoscopes according to the present invention is composed of lenselements in a number on the order of 2, and is nevertheless an opticalsystem which favorably corrects aberrations, lateral chromaticaberration in particular, and has high optical performance.

What is claimed is:
 1. A portable TV telephone, comprising: an objectivelens system configured to form an image of an object; an antennaconfigured to transmit and receive at least one of said image of theobject, a dial number and a voice carried by radio waves; and a displayconfigured to display at least one of said image of the object and thedial number, wherein said objective lens system comprises a firstnegative lens unit and a second positive lens unit in order from anobject side, wherein said objective lens system further comprises anaperture stop disposed between said first lens unit and said second lensunit, and wherein said first lens unit and said second lens unit eachhave a lens barrel configured to position the respective lens units suchthat circumferences of the lens units contact interior walls of theportable TV telephone.
 2. The portable TV telephone according to claim1, further comprising a switch configured to operate at least the dialnumber.
 3. The portable TV telephone according to claim 2, wherein saidfirst lens unit consists only of a negative lens element and said secondlens unit consists only of a positive lens element.
 4. The portable TVtelephone according to claim 2, wherein said second lens unit has anouter circumferential portion that has a cutout portion outside aneffective diameter through which rays transmit and said lens barrelpartially contacts said outer circumferential portion.
 5. The portableTV telephone according to claim 2, wherein said first lens unit consistsonly of a negative lens element and said second lens unit consists onlyof a positive lens element, and wherein said second lens unit has anouter circumferential portion that has a cutout portion outside aneffective diameter through which rays transmit and said lens barrelpartially contacts said outer circumferential portion.
 6. The portableTV telephone according to claim 3 or 5, wherein said first lens unit isconfigured to have an aspherical surface.
 7. The portable TV telephoneaccording to claim 6, wherein said aspherical surface is configured toweaken refractive power for rays as portions of the aspherical surfaceare distanced from an optical axis toward a marginal portion, saidoptical axis being positioned at a center of the lens unit.
 8. Aportable TV telephone, comprising: an objective lens system configuredto form an image of an object; an antenna configured to transmit andreceive at least one of said image of the object, a dial number and avoice carried by radio waves; and a display configured to display atleast one of said image of the object and the dial number, wherein saidobjective lens system comprises a first negative lens unit and a secondpositive lens unit in order from an object side, wherein said objectivelens system further comprises an aperture stop disposed between saidfirst lens unit and said second lens unit, and wherein said first lensunit and said second lens unit are cemented at outer circumferentialportions outside an effective diameter through which rays transmit. 9.The portable TV telephone according to claim 8, further comprising aswitch configured to operate at least the dial number.
 10. The portableTV telephone according to claim 3, 5 or 9, wherein said first lens unithas a planar surface on the object side.
 11. The portable TV telephoneaccording to claim 3, 5 or 9, wherein said aperture stop and said secondlens unit are cemented at outer circumferential portions outside theeffective diameter.
 12. The portable TV telephone according to any oneof claims 2 through 5 and 9, wherein said objective lens system isdisposed on a surface located on the same side as a surface on whichsaid image display is disposed.
 13. The portable TV telephone accordingto any one of claims 2 through 5 and 9, wherein said objective lenssystem is disposed on a surface located on the same side as a surface onwhich said switch is disposed.
 14. A portable TV telephone comprising:an optical system comprising a first lens unit; and a second lens unit,one of the said first and second lens units comprising a lens having atleast one aspherical surface, and wherein said first lens unit and saidsecond lens unit are cemented to each other outside an effectivediameter thereof.
 15. A portable data input unit comprising: an opticalsystem comprising a first lens unit; and a second lens unit, one of thesaid first and second lens units comprising a lens having at least oneaspherical surface, and wherein said first lens unit and said secondlens unit are cemented to each other outside an effective diameterthereof.